1. Verb - to press up against or to jostle.
2. Noun - the end part of a rifle or shotgun or machinegun that rests against the shoulder or pectoral muscles to increase stability during firing.
3. Noun - the part of a human being that knows wind and earth. The buttocks and anus of a person.
4. Noun - the recipient or target of a joke.
Ever since Father O'Malley was caught butting that butt into that young choirboy's butt, he has become the butt of many cruel jokes.
Two of the round cheeks below your back and above you thigh. Used to do things like fart and jiggle. There are other terms like booty, behind, cheeks, etc.