Frivolous pranks; japes and jests, etc: He was full of monkey business, the clown
Dubious and dishonest stratagems; trickiness: Show these kids that you're going to stand for no monkey business(1883+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with monkey business
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