ass: [OE] Ass comes ultimately from Latin asinus ‘donkey’ (whence English asinine ), and English probably acquired it via a Celtic route, from a prehistoric Old Celtic *as(s)in (source of Welsh asyn). As borrowed directly into the Germanic languages, by contrast, the n of Latin asinus changed to l; from this branch of the word’s travels Old English had esol, long defunct, and Dutch has ezel, which English has appropriated as easel. Further back in time the word’s antecedents are unclear, but some would trace it to Sumerian ansu, which could also be the source of Greek ónos (whence English onager ‘wild ass’ ) and Armenian eš. => easel, onager
beast of burden, Old English assa (Old Northumbrian assal, assald) "he-ass," probably from Old Celtic *as(s)in "donkey," which (with German esel, Gothic asilus, Lithuanian asilas, Old Church Slavonic osl) ultimately is from Latin asinus, which is probably of Middle Eastern origin (compare Sumerian ansu).
For al schal deie and al schal passe, Als wel a Leoun as an asse. [John Gower, "Confessio Amantis," 1393]
Since ancient Greek times, in fables and parables, the animal typified clumsiness and stupidity (hence asshead, late 15c., etc.). To make an ass of oneself is from 1580s. Asses' Bridge (c. 1780), from Latin Pons Asinorum, is fifth proposition of first book of Euclid's "Elements." In Middle English, someone uncomprehending or unappreciative would be lik an asse that listeth on a harpe. In 15c., an ass man was a donkey driver.
slang for "backside," first attested 1860 in nautical slang, in popular use from 1930; chiefly U.S.; from dialectal variant pronunciation of arse (q.v.). The loss of -r- before -s- attested in several other words (such as burst/bust, curse/cuss, horse/hoss, barse/bass, garsh/gash). Indirect evidence of the change from arse to ass can be traced to 1785 (in euphemistic avoidance of ass "donkey" by polite speakers) and perhaps to Shakespeare, if Nick Bottom transformed into a donkey in "A Midsummer Night's Dream" (1594) is the word-play some think it is. Meaning "woman regarded as a sexual object" is from 1942. To have (one's) head up (one's) ass "not know what one is doing" is attested by 1969. Colloquial (one's) ass "one's self, one's person" attested by 1958.
1. Just play it safe, cover your ass, keep your head down.
2. They've really been kicking ass lately — busting places up, harassing everybody.
3. He was generally disliked and regarded as a pompous ass.
4. Don't be such an ass!
5. The donkey is a domesticated form of the African wild ass.