The word is built in front of you: butt plus hole, the hole in the butt. That is the whole starting picture, and the rest of the word grows out of how the picture feels rather than what it is. Naming the least part of the body is already a small act of , and English keeps reaching for it whenever it wants to say that a person, or a place, is the worst of its kind.
這個字的組成就攤在眼前：butt（屁股）加 hole（洞），也就是屁股上的那個洞。起點的畫面就這麼簡單。後面的用法長出來的方式，靠的不是這個洞是什麼，而是講出它的那種感覺。點名身體最不體面的部位，本身就已經是一種粗魯。所以英語想說某個人、某個地方「爛到底」時，就一再回來借用這個畫面。
From the word’s history
The packet records the word simply as butt + hole, which is exactly what it still means in its first use.
資料顯示這個字就是 butt 加 hole 組成的，而它第一個意思到今天仍然完全等於這個字面組合。
Wiktionary, "butthole"
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