solid ungulate quadruped beast of burden of the horse kind, but smaller and with long ears and a short mane, native to southwest Asia, Old English assa (Old Northumbrian assal, assald) "he-ass." The English word is cognate with Old Saxon esil, Dutch ezel, Old High German esil, German Esel, Gothic asilus, and, beyond Germanic, Lithuanian asilas, Old Church Slavonic osl, Russian oselŭ, etc. All probably are ultimately from Latin asinus. De Vaan says the form of asinus suggests it was a loan-word into Latin, and adds, "Most IE words for 'ass' are loanwords."
Together with Greek onos it is conjectured to be from a language of Asia Minor (compare Sumerian ansu). The initial vowel of the English word might be by influence of Celtic forms (Irish and Gaelic asal), from Old Celtic *as(s)in "donkey." In Romanic tongues the Latin word has become Italian asino, Spanish asno, Old French asne, French âne.
In familiar use, the name ass is now to a great extent superseded by donkey (in Scotland cuddie); but ass is always used in the language of Scripture, Natural History, proverb, and fable; also, in ordinary use, in Ireland. [OED]
Sure-footed and patient in domestication, since ancient Greek times, in fables and parables, the animal has typified clumsiness and stupidity (hence ass-head, late 15c., etc.). To make an ass of oneself is from "A Midsummer Night's Dream" (1590). 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.
For al schal deie and al schal passe, Als wel a Leoun as an asse. [John Gower, "Confessio Amantis," 1393]