When you and your friends hide in a tree and jump down when people walk beneath you, scaring the crap out of them
Dude, remember that one time we were playing night monkey and I got chased?
Oh dude that was so funny!
a prank when you get in someones bed while they are (usually) sleeping and and jump up and down and basically hump them
Last night Matt night monkeyed me and it was quite awkward