to jump up and force a basketball down through the net in order to score: He passed the ball ahead to Mr. Love, who slam-dunked it. [ T ] informal to win something or defeat someone very easily, or to do something very successfully:

At its core, "slam dunk" is an expression used to signify an action or event that is virtually guaranteed to succeed or a situation where success is almost certain. This idiom is derived from the world of basketball, where a "slam dunk" refers to a player scoring by forcefully thrusting the ball through the hoop with one or both hands.

A forceful, dramatic move, as in That indictment was a slam dunk if ever there was one. This expression is also often put as a verb, slam-dunk, meaning “make a forceful move against someone,” as in This is a great chance for us to slam-dunk the opposition.