The storm 'threw a monkey wrench into' their plans for a picnic. [=the storm ruined their plans for a picnic]
After many years of disappointment, she finally 'got the monkey off her back' by winning the championship.
Our teacher warned us not to try any 'monkey business' while she was out of the room.
(..) and slept and slept, like she hadn't slept in years. It was wonderful having her home. Every day we'd take a walk, and I'd jabber on like a monkey in a tree, and she'd listen about ping-ponging and shrimping and Mama making a trip up to heaven. I did all the talking. Jenny most of the time (..)1994 Forrest Gump