"Monkey business" is an idiom. It describes an activity where one is up to no good, but is not doing anything too harmful. Monkey business does not cause you to get arrested but may provide a good laugh.
John: Can you let me borrow some cash? I'll pay you next week.
Ali: I don't think so, that sounds like monkey business (Ali doesn't believe that John will repay him if he lends him the cash).