Start with the body part the word literally names: the opening at the very bottom, the one nobody wants to look at or talk about. Every other use points back at that, and borrows the disgust rather than the anatomy. Aim it at a person and you get the insult; aim it at a place and you get the worst corner of it.
可以想成這個字照字面指的那個部位：身體最下面的開口，沒有人想看、也沒有人想談。其他用法都回頭指向這個畫面，借的不是解剖學，而是那份嫌惡。把它指向一個人，就成了罵人的話；把它指向一個地方，就成了那裡最爛的角落。
From the word’s history
Variant of earlier arsehole, from Middle English arshole, equivalent to ass + hole; cognate with Norwegian rasshol, Swedish arsle and German Arschloch.
這個字是較早的 arsehole 的變體，來自中古英文 arshole，等於 ass 加上 hole。挪威文 rasshol、瑞典文 arsle、德文 Arschloch 都是同一套組法。所以它的起點就是字面的身體部位，罵人的用法是後來長出來的。
Wiktionary entry for asshole, etymology 1 (kaikki:3f6f69a190036ae4:462869188), etymology_text supplied in the evidence packet
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