Parthian shot

Parthian shot [ˈpɑrθiən ʃɒt] n.

1.) A final hostile remark or gesture made while leaving (Microsoft Encarta College Dictionary).

Etymology: The Parthian horsemen were accustomed to baffle the enemy by their rapid manœuvres, and to discharge their missiles backward while in real or pretended flight: hence used allusively in Parthian fight, Parthian shaft, Parthian shot, Parthian glance, etc. (O.E.D. 2nd Ed.).'

"The climactic piece of dental-dramatic advice is the Old Man's Parthian shot: 'The next time, floss!' Indeed flossiness is all, and if you enjoy two and a half hours of smart-ass chatter and campy subversion—e.g., the honeymoon lament 'It's a real busman's holiday with you around: you could f--- up a wet dream!'—this may be your cup of…whatever ("Bodying Forth", John Simon, 1990).

(Defaite de Crassus, Augustyn Mirys, ~1750)