candyass

"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 not know one's ass from ___ "not know anything, be a fool," is by 1927 with from a hole in the ground; the other usual version being from (one's) elbow (by 1963). To not know an ass from a horse (or mule) is occasionally attested as a figure of ignorance from c. 1900.

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] 

Use as an emphatic (big-ass, bitch-ass, etc.) attested by 1953, originally African American vernacular, derived from earlier -assed in same sense attested by 1903.

late 13c., "crystallized sugar," from Old French çucre candi "sugar candy," ultimately from Arabic qandi, from Persian qand "cane sugar," probably from Sanskrit khanda "piece (of sugar)," perhaps from Dravidian (compare Tamil kantu "candy," kattu "to harden, condense").

The sense gradually broadened (especially in U.S.) to mean by late 19c. "any confection having sugar as its basis." In Britain these are sweets, and candy tends to be restricted to sweets made only from boiled sugar and striped in bright colors. A candy-pull (1865) was a gathering of young people for making (by pulling into the right consistency) and eating molasses candy.