Etymology
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arse (n.)

"buttocks, hinder part of an animal," Old English ærs "tail, rump," from Proto-Germanic *arsoz (source also of Old Saxon, Old High German, Old Norse ars, Middle Dutch ærs, German Arsch "buttock"), from PIE root *ors- "buttock, backside" (source also of Greek orros "tail, rump, base of the spine," Hittite arrash, Armenian or "buttock," Old Irish err "tail").

To hang the arse "be reluctant or tardy" is from 1630s. Middle English had arse-winning "money obtained by prostitution" (late 14c.). To turn arse over tip is attested by 1884, along with the alternative arse over tit.

Every scrap of Latin Lord Edgecumbe heard at the Encaenia at Oxford he translated ridiculously; one of the themes was Ars Musica : he Englished it Bumfiddle. [Horace Walpole to the Countess of Upper Ossory, Aug. 9, 1773]
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arsy-versy (adv.)
"backside foremost," 1530s, probably a reduplication of arse, perhaps with suggestions of reverse.
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arsehole (n.)

c. 1400, arce-hoole; see arse + hole (n.). In Old English, Latin anus was glossed with earsðerl, literally "arse-thrill," with thrill (n.) in its original sense of "hole" (compare nostril).

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wheatear (n.)
type of bird, 1590s, back-formation from white-ears, literally "white-arse" (see white + arse). So called for its color markings; compare French name for the bird, cul-blanc, literally "white rump."
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cynosure (n.)

"something that strongly attracts attention," 1590s, from French cynosure (16c.), from Latin Cynosura, literally "dog's tail," an old name of the constellation (now Ursa Minor) containing what is now (but was not in ancient times) the North Star, the focus of navigation, at the tip of its tail; from Greek kynosoura, literally "dog's tail," from kyōn (genitive kynos; from PIE root *kwon- "dog") + oura "tail" (see arse). Apparently in ancient times the whole constellation was used as a rough indicator of the celestial north pole. Related: Cynosural.

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squirrel (n.)

early 14c., from Anglo-French esquirel, Old French escurueil "squirrel; squirrel fur" (Modern French écureuil), from Vulgar Latin *scuriolus, diminutive of *scurius "squirrel," variant of Latin sciurus, from Greek skiouros "a squirrel," literally "shadow-tailed," from skia "shadow" (see Ascians) + oura "tail," from PIE root *ors- "buttocks, backside" (see arse). Perhaps the original notion is "that which makes a shade with its tail," but Beekes writes that this "looks like a folk etymology rather than a serious explanation." The Old English word was acweorna, which survived into Middle English as aquerne.

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defecation (n.)

1640s, "purification of the mind or soul" (figurative); 1650s, "act or process of separating from lees or dregs, a cleansing from impurities," from Late Latin defecationem (nominative deficatio), noun of action from past-participle stem of Latin defaecare "cleanse from dregs, purify," from the phrase de faece "from dregs" (see de- + feces). Meaning "act of evacuating the bowels" is from 1830. An Old English word for "bowel movement" was arse-gang literally "arse-going."

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