Why are the "donkey" and the "butt" both named "ass"? [closed]

Is there any similarity between them that they have the same name, or is the reason something different of having a physical similarity?

I found different meanings for both, but none of them clarified the matter of naming them the same in vulgar English speaking.

It's a historical accident—they’re really two different words.

In the sense buttocks, the word goes back to OE ærs, and beyond that to Proto Indo-European: there are cognates in Greek, Hittite and Old Irish. This is reflected in the ordinary British English arse—the {r} is dropped only in US English.

In the sense donkey, the word goes back to OE assa, derived (it is thought) via Celtic from Latin asinus (which may itself be of Middle Eastern origin).

See the Online Etymological Dictionary, s.v. arse and ass, and the Oxford English Dictionary, s.v. arse and ass.

Butt was originally (and in British English still is) arse. The donkey is ass. In American English these words merged into a single ass. The etymology has nothing to do with the similarity of the animal to a behind.