monkey business
monkey business {n.}, {slang}, {informal}
1. Any unethical, illegitimate, or objectionable activity that is furtive or deceitful, e.g., undercover sexual advances, cheating, misuse of public funds, etc.
There is a lot of monkey business going on in that firm; you'd better watch out who you deal with!
2. Comical or silly actions; goofing off.
Come on boys, let's cut out the monkey business and get down to work!
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Source: A Dictionary of American Idioms