monkey business

WordReference Random House Learner's Dictionary of American English © 2025

ˈmon•key ˌbusi•ness, n. [uncountable]

1. foolish, playful, or mischievous behavior:
   schoolboys up to some monkey business.
2. dishonest conduct;
   trickery.

WordReference Random House Unabridged Dictionary of American English © 2025

mon′key busi′ness, 

1. frivolous or mischievous behavior.
2. improper or underhanded conduct;
   trickery.
• 1880–85, American.

Collins Concise English Dictionary © HarperCollins Publishers::

monkey business noun

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