to get angry; to become irritated
Tom a commencé à s'énerver.
Tom started to get annoyed.
Ce type-là est charmant, mais je l'ai vu s'énerver.
That guy is charming, but I’ve seen him lose his temper.
— Gramps Is in the Resistance (1983)
Il aurait mieux fait de se taire au lieu de s'énerver.
He would have been better off keeping quiet instead of getting angry.
Il n'y a pas de raison de s'énerver.
There's no reason to get upset.
I bet he's going to get angry.