sit on ass
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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.
sit on one's ass
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.