something that is a sure to occur; a foregone conclusion; "predicting his success was a slam dunk"

a forceful dunk

see dunk.

a type of dunk where the ball is thrown down hard through the basket.

Slam Dunk (スラムダンク suramudanku) is a 31-volume Japanese manga series written by Takehiko Inoue about a basketball team from Shohoku (Shōhoku) High School. It has sold over 100 million copies in Japan alone.