Slam-dunk

A slam-dunk, first cited from the 1970s, is the forceful slamming of a basketball down into the net, an American sporting expression that has since developed the figurative meaning of a foregone conclusion or a sure thing, which dates from the 1980s. The origin of the expression is connected to dunk, the submerging of a biscuit or doughnut into a hot beverage before eating, in much the same way as dunking a basketball into the net.