To date, Leafoosish is my only conlang that technically has "bad" words. Historically, Tikolmian did too (conhistory), but now it doesn't anymore because they're too bad.
In Leafoosish,
conell is a common interjection/curse. The literal meaning of it is "rabbit", but it derives from the phrase
la gonell dell mall (literally, "the rabbit (fem.) of evil";
gonell is just soft-mutated from
conell), which means "epilepsy". In my conworld...well, maybe I shouldn't explain.

Tikolmians once also invoked "the Rabbit", as it's known in my conworld, in vain, but they've decided not to do that anymore because they think she'll hear her name and decide to, you know, stop by. These days, Tikolmian uses
takrielel ("guinea pigs"; literally, firemice) as an interjection, and the phrase
via zo takrielel-mi "go to the guinea pigs" is roughly equivalent to "go to hell" but without such a clear meaning. If a Tikolmian speaker really feels like being rude, s/he can also say
zo tuqsiōl sod-caz rōma "may epilepsy meet with you", but this is not recommended as you will probably be given a permanent silent treatment or at least an ear-piercing session of yelling.
I know, I'm very weird.

This is what you get if you ask me to discuss swear words in my conlangs.