You can think of arse as the plain, rude name for the part of the body you sit on — the lowest and least part of a person. Nearly everything else the word does can be seen growing out of that one picture. It is the part that stays on the chair when someone will not get up. It is the part you name when you want to shrink a whole person down to something worthless. And it is what you reach for when you want to say that something is as bad as it gets.
From the word’s history
Wiktionary traces arse through Middle English ars/ers and Old English ærs to Proto-Germanic *arsaz and the Proto-Indo-European root *h₁órsos, meaning 'backside, buttocks'. German Arsch and Dutch aars come from the same root. So the body-part picture is not a later development — it is where the word starts.
依照 Wiktionary 的說明，arse 從中古英語 ars／ers、古英語 ærs 一路上溯到原始日耳曼語 *arsaz，最遠到印歐語根 *h₁órsos，意思就是「後面、屁股」。德文的 Arsch、荷蘭文的 aars 同源。也就是說，身體部位這個畫面不是後來才加上去的，它本來就是這個字的起點。
Wiktionary entry 'arse' (packet source_ref kaikki:3f6f69a190036ae4:502950985)
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