One way to hold this word: a girl's name borrowed as a cover for a body part nobody wants to name directly. Because it starts as a name, it sounds light and harmless — and that is exactly the problem, because the two sides of the Atlantic put that light-sounding name on two different parts of the body.
Wiktionary dates the body-part use to the 19th century and traces it to Fanny, the pet form of the name Frances. The same etymology group covers the buttocks sense as well as the British one.
根據 Wiktionary，這個身體部位的用法出現在十九世紀，來自女子名 Frances 的暱稱 Fanny。同一個來源底下也包含了「屁股」這個意思。
Wiktionary (kaikki:3f6f69a190036ae4:958125777): "19th century. Ultimately from Fanny, pet form of the name Frances. Compare dick, John Thomas, mickey."
One branch
同一個名字，兩個地方，兩個部位
兩個意思共用同一個來源和同一種「不直說」的做法，差別只在地理：美加指一個部位，英澳紐愛指另一個。
noun · B1
The part of your body that you sit on. In American and Canadian English this is a mild, gentle word for it — the kind adults use with children, or in light, joking talk among friends.
屁股