una parolaccia
In Italian, a bad word is una parolaccia.
The noun parolaccia is built from parola (word) and the pejorative suffix -accia, so it literally means an ugly word. A common example is stronzo, which roughly translates to 'a**hole' in English.
How does it change in a sentence?
Parolaccia is feminine. It takes una in the singular, and the plural is parolacce, as in dire le parolacce (to swear).
Example conversation
Giulia and Alessandro are talking about a movie they just watched.
- GiuliaIl protagonista ha detto una parolaccia!The main character said a bad word!
- AlessandroSì, non era molto educato.Yes, he wasn't very polite.
- GiuliaNon mi piace sentire una parolaccia.I don't like hearing a bad word.
- AlessandroHai ragione, Giulia.You are right, Giulia.