the.com/slam dunk

means A basketball shot where the player jumps and forces the ball down through the hoop by hand, and more broadly, any sure thing or guaranteed success.

from Both halves are plain English doing honest work: 'slam' for the forceful, crashing motion, and 'dunk' from the same word used for dunking a biscuit in coffee — dipping something down into something else. The basketball move was being described this way by mid-20th-century players and broadcasters, and by the 1970s-80s 'slam dunk' had bounced out of the sport entirely, becoming American slang for any certainty — a decision, a deal, a bet — so obviously correct it needs no more debate than a ball already through the net.