Ass job is a term for a sexual technique in which an individual masturbates their partner by placing their penis between their intergluteal cleft, known colloquially as the butt crack or ass crack.
The term takes its name from the colloquial term "hand job" which refers to a similar masturbation technique using the hand instead of the intergluteal cleft.
Ass job is sometimes written as one word, assjob. An ass job is also known as butt cheek sex, a butt job, and a hot dog.