Cuddlebum
When you are cuddling up against someone's butt (i.e. spooning), then that person is your Cuddlebum.

This term came to fruition and gained popularity from the show Will & Grace when one of the characters asked "Will you cuddle me bum?" and the nickname Cuddlebum was then given.
RANDOM GIRL: I'm lonely and cold; I need someone to cuddle me bum!

RANDOM BOY: I'm lonely. I wish I had a Cuddlebum!

(**BOY and GIRL hear each other, look at each other, sparks fly. They get in bed together and spoon**)