But we're talking about another explanation of the meaning of the word. Monkey can also be used as an exclamation, like the word blimey. Blimey is used when something happens suprisingly, like when someone's getting jumped from behind.
Monkey is 100 % certainly evolved by a fifteen year old student in Norway. According to the rumours he was playing a computer game, and when things went wrong, he cried "Monkey!".