The monkey in the middle is the person that is in the middle of two fighting sides. This person is friends with both arguing sides and wants to stay neutral but is eventually dragged into the fight, and one of the fighting sides become mad at them.

when you're hungover as shit after a spending a night drunk as a skunk

A sex position where a whale is on top of a monkey and he fucks the whale's buns while the monkey is on top of SpongeBob while he gets butt fucked by his SpongeKnob SquareNuts. Long story short, the monkey is in the middle.