sit on (one's) ass

rude slang To be idle, especially when action is needed. You can't just sit on your ass all summer. Get a job, join a club—do something with your time! I was tired of sitting on my ass while so many people in the city needed help, so I decided to join a homeless shelter and try to lend a hand where I could.

Inf. to sit idle; to sit around doing nothing. (Use ass with discretion.) Don't just sit on your ass! Get busy! He just sat on his ass, watching what was going on.

Be extremely lazy, do as little as possible or nothing at all. This is a ruder version of sit on one’s hands, which dates from the early 1900s and is not heard much any more. The somewhat newer expression appears in numerous contexts, as, for example, “This new president is much nicer than his predecessor, but so far he’s just sitting on his ass and collecting his stipend.” Sometimes “butt” is substituted for “ass.”