monkey business 
noun [mass noun] informal
Mischievous or deceitful behaviour: she wasn’t going to have any monkey business where the reputation of her only daughter was concerned...
- It seems that 2003 is off to a good start as, this week, DJs big and small will provide many a chance for tomfoolery and monkey business.
- I think that when you have the television screens in the courtroom, less monkey business goes on.
- When you look at these pictures, you know there was no monkey business, and that I was not sneaking around trying to steal pictures of people.
Synonyms
mischief, misbehaviour, mischievousness, devilry, devilment, rascality, tomfoolery;
dishonesty, trickery, misconduct, misdemeanour, chicanery, skulduggery
informal shenanigans, funny business, hanky-panky, goings-on
British informal monkey tricks, carry-on, carryings-on, jiggery-pokery
North American informal monkeyshines