What is the spanish word for butt?

Depends on where the person is from.... These are all slang terms and some can mean other things to different people.

Fundillo (Back side)

Colilla (Little tail - could mean a female body part in some areas, or a cigarette butt in others)

Tocón (Stump)

Culo (Vulgar - can translate to Ass or even Ass H...)

This one is tricky. The best suggestion here is not to use this term unless you know which one is correct in your particular situation.