A list of various insults in Esperanto that I've found useful in things like translations and fiction. This is certainly not a complete list, and of course, anyone can make up their own insults too. For help on how to write casual (vulgar, low-brow, lower-class) speech in general in Esperanto, see my other page about dialects and socialects.

To insult someone, you simply use the normal form as in English (stultulo! = idiot!), but you can also make it more of an exclamation by using only the stem (stultul! = idiot!). Normally exclamations like "fek! shit, fuck!" and "ek! let's go, let's start!" don't have apostrophies, but when normal words are "slurred" then they do have them ("tia stultul' estas..." "that sort of idiot is..."), so keep that in mind. In general, the stems without an ending are used for things like exclamations and sound effects. However I've noticed that when translating comics, it's best to include the -o when the character is talking to another person who is a bit further away (implying "what an idiot!" or "that idiot over there...!") but when talking to their face right on front of them we can use only the stem if we want. There's no need to say the "you" in "you idiot" because it's obvious who is being talked about.

There's three major prefixes that can be used to help form a whole lot of insults:

1. fi means "fie; shameful, immoral, non-noble", as in "fimagio = black magic" or "fi! - fie!" or "fikomercisto - swindler, conman, corrupt businessman". In general this simply adds force to insults. It can go attached in front of the thing (fikomercisto) or separately (fi komercisto) or as an adjective (fia komercisto). To say like "why, you...!" we can say "fia ci!" or something similar.

2. "kvazaŭ - quasi, pseudo, semi-, as though, as if", as in "kvazaŭina - quasi-female (shemale, tranny, etc.)".

3. "mal - opposite (un-, non-, im-)", for example "malanĝela - "opposite-of-angelic; devilish". This can sometimes give a different flavour to the word, ex. with "ino = a female", the normal word for a male is "viro", but we can say "malino (opposite-of-female)" to somewhat further emphasize the difference between male and female. This can feel like if we were to say "you men are all pigs!" and stress "men". Not everyone will see them as at all different from each other, but some people will.

Keep in mind that something which is insulting or humiliating in one country might not be in another. For example, saying "you monk!" doesn't even feel like an insult in English; pink is not only a colour for girls in Sweden and Japan; in Nordic countries beards are more like classy and elegant instead of only for hobos, drug addicts and pedophiles. Calling a guy a "stud" might be a huge insult because it's literally the male equivalent of "slut", (somehow in English we think "stud" is a compliment whereas "slut" is an insult...?). But of course there are still many things we can say that avoid those kinds of cultural differences.

Esperanto English
kvazaŭedzino "quasi-wife", concubine, mistress. it's good to use this when one character is making fun of a couple for acting like they're married, for example.
malvirta "un-virtuous", of very low moral standards, corrupt
"brain atrophied",
vegetable, demented, comatose etc.
(-ita or -ata is just according to preference)
pepi, pepadi "to chirp; to keep on chirping",
to babble annoyingly (ex. tittering)
cease to live!; without-life become!;
dead-become! (all meaning "go die!")
"un-chaste-person-ess", "pit/well-ess",
a slut, whore.
fek! fuck! shit!
histrikino "porcupine-ess", a difficult woman
dickless; without a penis.
"kaco = cock", "penis = penis", so kaco is more rude.
svatisto "match-maker" (for marriage).
apparently a euphemism for "pimp".
ŝtelaĵkaŝisto "stolen-things-concealer-for-job",
a person whose job it is to hide stolen items
forputru! putru for! "putrify away! rot away!"
go to hell! fuck off!
filiteraturo; fihistorio "immoral literature"; "immoral story/history",
smut, criminal history, etc.
malista "un-professional", amateaurly
(without clear context, this could be taken to mean "has something to do with 'opposites' for their job")
kvazaŭino "quasi-female", shemale, tranny, etc.
(sometimes simply translated as "faggot")
kvazaŭhomo "quasi-human", manikin, doll, so worthless, disgusting or apathetic that they're not even human, etc.
nevivinda "not-life-worthy",
not worthy of being alive
rampulo; fiaĉulo a creeper, crawler (person who does nothing but sneaks around); "ignoble-bad-quality-person", a thoroughly awful creep or disgusting person
urintrinkulo; pistrinkulo urine-drinker; piss-drinker
(I recommend "pistrinkulo" because it is easier to say aloud)
insect-eaten; butterfly (moth)-eaten; mouse-eaten;
means "useless" or "no good",
ex. “I’ve thought of a thousand ideas, and every one’vem is moth-eaten.”
"miss (wrongly)-nursed",
ill-bred; deformed person; with abusive parents (missfoster på svenska)
malsaĝa, stulta unwise, stupid
petisto "request-for-job", a begger; someone who does nothing but ask for things from others
perfida perfidious, untrustworthy, deceitful, traitorous
-am(or)ema; -am(or)emulo, -(al)voluptema
(bestamemeco, best(al)voluptema)
-philia, -philiac, etc.
"animal-love-inclination-ness", "animal-(towards)-lasciviousness", bestiality, zoophilia. "am" is like "deep love", whereas change it to "amor" would be "sex".
bubo, knabaĉo "wicked boy", "boy-bad-quality", brat
forgesindulo "forget-worthy-person", a nobody
krimetemulo; bandetaĉano; fiaretano; fibandetano "small-crime-inclined-person"; "member of a bad-quality and small gang/troop"; "ignoble member of a small group"; "ignoble member of a small gang/troop". delinquent, hoodlum, small-time gangster or criminal.
Note that putting "fia (fia bandetano)" before as an adjective describes the person as ignoble, where as compounding it like "fi- (fibandetano)" describes the group/gang as ignoble.
akonita aconite (storm-hat, monk's hat, wolf's tooth).
a very poisonous, purple-ish flower. could easily be used to describe a woman or something else which is "beautiful but deadly".
akretema acrimonious (angry and bitter)
maladepta inept
malsufiĉulo "un-sufficient-person", an inadequate person
naiva naïve (simple, innocent)
jesulo a yes-man (someone who does nothing but agree with others)
hontindulo "shame-worthy-person", a person to be completely ashamed of
maldiligentulo "un-diligent-person", a loafer, sloth, lazy person
malhelpo an "un-help", a hinder, obstacle
malvigla, malatentema "un-vigilant", "un-attentive-inclined"
neglectful, negligent, not alert, not paying attention
malĉarma not charming, unattractive
dando a dandy, a shallow person (someone too concerned with looking fashionable)
metiisto blue-collar worker, day-laborer, manual worker
avara avaricious (having extreme greed for wealth or material gain)
malplibonigi; plimalbonigi to "un-ameliorate", to cause something to become worse. we can easily turn this into an insult like "he's someone who only ever has bad ideas" etc.
amoristo, amorvendisto, sinvendisto, ĉiesulino, putino "amorous-for-job", "sex-sell-for-job", "self-sell-for-job", "everyone's-person-ess", "well/pit-ess",
a player, wooer, prostitute, sex-worker. I would also use this as an insult for the Japanese-style "hosts".
"wilderness/desolate-person", an anchorite (a religious recluse), a hermit
ŝeleca, membraneca "shell/skin/bark/rind-like", "membrane-like", shallow, skin-deep. "membraneca" also gives a little disgusting feeling.
diafanulo, travideblulo "pellucid-being", "through-visible-being", a transparent person, someone who's easily seen through, someone who's existence is so thin that they're easily forgettable/ignorable, etc.
penetrilo, infanigilo, kaco, peniso "penetrate-tool (penis)", "child-cause-tool (genitals)", cock, penis.
there are many more terms for this.
pendol(em)ulo, nedecid(em)ulo "pendolum-(inclined)-person", "not-decide-(inclined)-person", a waffler, a wishy-washy person.
(note that "pendolum..." can also mean "a commuter")
malestro "opposite-of-boss", meaning "the lowest-ranked person"
ek(em)ulo "brief/begin-(inclined)-person", a poser (someone who's only into a subject for a brief time); someone who starts things/projects and then drops them; a n00b, newbie, novice.
avidulo "avid person", someone who has a keen craving or large desire for something (sometimes can be translated as "desperate/greedy person")
peritoneo peritoneum (the serous membrane lining the cavity of the abdomen and covering the abdominal organs). could simply be used to form compounds like "a shield, a whiteknight" or "disgusting person" depending on context.
piulo a pious (strongly religious) person. this could be used like "a zealot" depending on context.
fortranĉita balano "severed-penis-head". (my friend suggested this insult. balano = the bulb-like area of the penis)
viraĉo "bad-quality male", bastard, son of a bitch, jerk, asshole.
bastardo a bastard - in the sense of "illegitimate child; a person born of parents not married to each other."
inaĉo, virinaĉo "bad-quality-female", bitch. for an extra punch (that some might not get, but some might feel extra offended by) use "virinaĉo" because it derives from "viro - male" and some people think that's more demeaning.
amoraĉetisto "sex-buy-for-job". a person who buys sex as their job, or buys so much that it could be considered their job.
farmaĉofilo, farmaĉido "bad-quality farm son", "bad-quality-farm-offspring".
this is probably a bit stronger than "were you born in a barn?!". I would use it to literally translate the Japanese insult "yarou (やろう, 野郎)".
fuŝulo "botch/bungle-person", a good-for-nothing, a clumsy person.
volupt(em)a, diboĉa, malĉasta "lascivious(-inclined)", "ribald, debaucherous, wanton", "un-chaste"
lewd, horny, perverted. (feeling or revealing an overt sexual interest or desire). adding "dis- (dispersed, apart)" will make these words mean "(widely) promiscuous".
People from the USA used to be very creative with insults. However in the past 20-30 years this has absolutely, completely died out. If you think that Americans are witty today, try watching some movies from the 1930's. The problem is that generic insults are always used nowadays, and this also bleeds into their Esperanto usage - they only insult with "fek! (shit! fuck!)" or "stultulo! (idiot!)" and can't think up anything else to say. It certainly makes reading bland!

So here I have collected some insults from earlier time periods, as examples to help stir the imagination:

— "he's only a habit"
(I don't actually like him, but I'm with him/dating him because I'm used to it, that's all)

— “he’ll wind up jerking sodas when he runs out of ideas”
(he will only be able to get a job at a fast-food restaurant, eventually)

— “Say, what’s the name of that guy who built the monster he couldn’t stop?”
“Shake friends with his Aunt Emma”
(meaning ‘greet this one annoying guy’s annoying aunt, who is named Emma’)

— “(Go) Outside. As long as they’ve got sidewalks, you’ve got a job.”
(telling a woman to leave, and saying that she's a slut/hooker)

— “Is there anything I can do (to help)?”
“Yeah. See that window over there?”
“Take a running jump and I think you can make it.”
(go jump out the window and die and leave me alone)

— “C’mon, breeze! And when ya go out that door, take the welcome sign off it.”
(leave! and don't come back!)

— “Seeing all these girls gives me a lotta ideas.”
“Well don’t let them keep ya awake.”
(the second guy is joking/hinting that the first guy is going to masturbate or that he is a pervert)

— "Previous writers had left Japanese grammar a chaos. Mr. Aston brought light and order into is every part. But most persons have neither time nor inclination to investigate every part."

— "Numerous other instances of this most comprehensive word might be given, but the intelligent student will at once understand the full value of a word of so much significance and bury it deeply within the inmost cell of his penetrating brain."

— "The more serious and scholarly man, who abounds in every club, is addicted to the monthly reviews, to the Times, to the Spectator; that kind of man is definitely stodgy and prides himself upon being sound. He is fond of memoirs, rather sodden accounts of aristocrats and politicians, of the dull, ordinary lives of dull, ordinary people; when he has done with the book it goes to the pulping machine, but some of the pulp gets into that man's brain."

— An old insult for "dimestore novel" (pulp fiction, trashy novel) was "railway novel" because it "cannot hold the attention for more than the time that elapses between two railway stops".