merda / merda - another word for shit. This one is still understood and used by Romance speakers, and depending on the language it’s pronounced almost the same, [mɛɾ.dɐ], so discretion is advised. You can also use it to refer to something that is crap, Martial did this for example.
lupa / lupa - she-wolf; slang for prostitute.
filius lupae / filī lupae - son of a she-wolf/bitch.
uerpus / uerpe - of a penis, showing the glans. When used as a noun it typically refers to either someone who’s shameless / a horny dog or circumcised.
stultus / stulte - dumb
asinus / asine - donkey
scelus / scelus - wicked, heinous
luteus - [you’re] filthy. In 2025 that doesn’t seem like a big deal but you’d get yourself killed in Roman times for that.
Iuppiter te perdat! - may Jupiter destroy you. Roughly “get damned”.
A few additional ones. I’ll list them in the nominative and then vocative (vocative makes more sense for insults).