A parting shot - Idioms by The Free Dictionary
a parting shot
A final, usually critical remark made specifically to have an impact on the listener(s). He couldn't resist getting a parting shot in at the hiring manager as he stormed out of her office.
a ˌparting ˈshot (also a ˌParthian ˈshot) a remark or action, often an unkind one, that somebody makes just as they are leaving: As Jim walked out of the door, his parting shot was, ‘I never want to see any of you again.’Parthia was a kingdom in ancient times. The Parthians used to fire arrows at the enemy as they were retreating from battle.