1
general::
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. The idiom comes from basketball, where it refers to a dramatic shot in which the ball is thrust into the basket from above the rim. It was transferred to other activities from about 1980 on.
American Heritage Idioms
2
general::
Phrase(s): slam dunk
1. Lit. [in basketball] a goal scored by shooting the ball down from above the rim. • He was wide open and scored on an easy slam dunk. 2. Fig. an action or accomplishment that is easily done. • Finishing that project with all his experience should be a slam dunk for George.
McGrawhill's American Idioms And Phrasal Verbs