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.
(..) in a public place for the purpose of commercial enterprise. Commercial enterprise? Yes. You play that thing and people give you the money. People give the monkey the money. - It is the same. - Oh, not at all, monsieur. I am a musician, and the monkey is a businessman. He doesn't tell me what to play, and I don't tell him what to do with his money. Monsieur, don't try to be funny with me. He is your monkey, therefore it is your money. He lives with me, but he is not my monkey. One day I came home, and I found him sitting in the living room. (..)1975 The Return of the Pink Panther