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.

slam dunk 1 noun Basketball. a particularly forceful, often dramatic dunk shot. Slang. something regarded as certain to occur or be accomplished, typically something desirable that requires little further effort: The election is starting to look like a slam dunk for our side.