monkey business

noun

1. frivolous or mischievous behavior.
2. improper or underhanded conduct; trickery.

noun

1. informal mischievous, suspect, dishonest, or meddlesome behaviour or acts

Silly, mischievous, or deceitful conduct, as in The teacher told the children to cut out the monkey business and get to work, or I don’t trust that lawyer—there’s some monkey business going on. This expression transfers the tricks of monkeys to human behavior. [Late 1800s]