bunyoudaoicibaDictYouDict[bun 词源字典]
bun: [14] The word bun first crops up in 1371, in an Anglo-Latin document relating to different types of bread. Its origins, however, are completely shrouded in mystery. Equally obscure, but presumably unrelated, is another word bun, which in the 16th century meant ‘squirrel’. By the 19th century we find it being used for ‘rabbit’, and it survives in its familiar form bunny.
[bun etymology, bun origin, 英语词源]
bun (n.)youdaoicibaDictYouDict
late 14c., origin obscure, perhaps from Old French buignete "a fritter," originally "boil, swelling," diminutive of buigne "swelling from a blow, bump on the head," from a Germanic source (compare Middle High German bunge "clod, lump"), or from Gaulish *bunia (compare Gaelic bonnach). Spanish buñelo "a fritter" apparently is from the same source. Of hair coiled at the back of the head, first attested 1894. To have a bun in the oven "be pregnant" is from 1951.

The first record of buns in the sense of "male buttocks" is from 1960s, perhaps from a perceived similarity; but bun also meant "tail of a hare" (1530s) in Scottish and northern England dialect and was transferred to human beings (and conveniently rhymed with nun in ribald ballads). This may be an entirely different word; OED points to Gaelic bun "stump, root."