ass (n.1)

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In familiar use, the name ass is now to a great extent superseded by donkey (in Scotland cuddie); but ass is always used in the language of Scripture, Natural History, proverb, and fable; also, in ordinary use, in Ireland.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, yet 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]
Related entries & more 
ass (n.2)
Origin and meaning of ass

"backside," attested by 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- is not uncommon (burst/bust, curse/cuss, horse/hoss, barse/bass, garsh/gash, parcel/passel).

Indirect evidence of the change from arse to ass can be traced to 17c. By 1680s arse was being pronounced to rhyme with "-ass" words, as in "Sodom or the Quintessence of Debauchery": "I would advise you, sir, to make a pass/Once more at Pockenello's loyal arse." It is perhaps as early as Shakespeare's day, if Nick Bottom transformed into a donkey in "A Midsummer Night's Dream" (1594) is the word-play some think it is.

I must to the barber's, mounsieur; for me thinks I am marvellous hairy about the face; and I am such a tender ass, if my hair do but tickle me, I must scratch. [Bottom]

By 1785 polite speakers were avoiding ass in the "donkey" sense. 

The meaning "woman regarded as a sexual object" is by early 1940s (piece of ass seems to be implied in 1930s Tijuana Bibles), but the image is older (compare buttock "a common strumpet," 1670s). 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. To work (one's) ass off "work very much" is by 1946; to laugh (one's) ass off "laugh very much" is by 1972 (implied from 1965). The (stick it) up your ass oath is attested by 1953; apparent euphemisms suggest earlier use:

He snoighed up his nose as if th' cheese stunk, eyed me wi an air o contempt fro my shoon to my yed, un deawn ogen fro my yed to my shoon ; un then pushin th' brade un cheese into my hont ogen, he says "Take your vile bread and cheese and stick it up your coat sleeve, and be demmed to you. Do you think I want your paltry grub?" Un then, turnin on his heel, he hurried into th' perk. ["Bobby Shuttle un His Woife Sayroh's Visit to Manchester," 1857] 
Related entries & more 

also smartass, 1960 (adj.), 1962 (n.), from smart (adj.) + ass (n.2).

Related entries & more 
raggedy-ass (adj.)

"rambling, straggling, disreputable," by 1930, from raggedy + ass (n.2). Such groups were known as raggle-taggle (adj.) by 1904.

Related entries & more 
bad-ass (n.)

also badass, "tough guy," 1950s U.S. slang, from bad (adj.) + ass (n.2).

Related entries & more 
dumb-ass (n.)

also dumbass, "stupid, ineffectual person," by 1959, from dumb (adj.) "stupid" + ass (n.2).

Related entries & more 
ass-head (n.)

also asshead, "stupid person, dullard," late 15c., asse hede, from ass (n.1) + head (n.). Related: Ass-headed.

Related entries & more 
hard-ass (adj.)

"tough, uncompromising," 1961, from hard (adj.) + ass (n.2). Probably originally military. As a noun, "tough, uncompromising person," from 1967. Old Hard Ass is said to have been a nickname of Gen. George A. Custer (1839-1876) among his cavalry troops because of his seeming tirelessness in the saddle.

Related entries & more 
ass-hole (n.)

also asshole, 20c., perhaps 1880s, American English variant of arsehole (also see ass (n.2)). Meaning "contemptible person" is from mid-1930s. Earlier the word was a Northern English and Scottish dialectal variant of ash-hole "receptacle for ashes beneath a grate." Wright's "English Dialect Dictionary" (1898) lists many examples, such as, "Tell'd her a hunderd times nivver to put t'poaker i' t'ass-hoil" [West Yorkshire].

Related entries & more 
assify (v.)

"to make an ass of" (someone), 1804, from ass (n.1) + -ify. Related: Assified; assification (1823).

Related entries & more