slam dunk • n. Basketball a shot in which a player thrusts the ball forcefully down through the basket. ∎ [usu. as adj.] inf. something reliable or unfailing; a foregone conclusion or certainty: a movie predicted to be the season's one slam-dunk hit.• v. (slam-dunk) [tr.] thrust (the ball) forcefully down through the basket. ∎ inf. defeat or dismiss decisively: they continue to slam-dunk every proposal we make.