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**)