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 a group of students sit down for playtime in kindergarten and the teacher assigns each kid to sit where, she sits a white kid down and then a black kid next to them, and then sitting another white kid next to the black kid, presenting monkey in the middle to racists.

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.