What is the meaning of the word "butt"?

Definitions:

the fleshy part of the human body that you sit on noun
- Their butt was sore from the long run.
to lie adjacent to another or share a boundary verb
- Canada's border to the south butts the United States.
to place end to end without overlapping verb
- I butted the frames at the joints.
to strike, thrust or shove against verb
- I butted my sibling out of the way.