Etymology
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moo (v.)

"to make the characteristic cry of a cow, to low," 1540s, of imitative origin (compare Latin mūgire "to low, moo," Lithuanian mūkiu "to bellow," Middle High German mūhen "to low, bellow," and see PIE root *gwou-). Related: Mooed; mooing. The noun "the low of a cow" is from 1789. Baby-talk moo-cow (n.) "a cow" is attested from 1812.

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low (v.)
Old English hlowan "moo, make a noise like a cow," from Proto-Germanic imitative *khlo- (source also of Middle Dutch loeyen, Dutch loeien, Old Low Franconian luon, Old High German hluojen). This is perhaps identical with the imitative PIE root *kele- (2) "to shout."
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moon (n.)

"heavenly body which revolves about the earth monthly," Middle English mone, from Old English mona, from Proto-Germanic *menon- (source also of Old Saxon and Old High German mano, Old Frisian mona, Old Norse mani, Danish maane, Dutch maan, German Mond, Gothic mena "moon"), from PIE *me(n)ses- "moon, month" (source also of Sanskrit masah "moon, month;" Avestan ma, Persian mah, Armenian mis "month;" Greek mene "moon," men "month;" Latin mensis "month;" Old Church Slavonic meseci, Lithuanian mėnesis "moon, month;" Old Irish mi, Welsh mis, Breton miz "month"), from root *me- (2) "to measure" in reference to the moon's phases as an ancient and universal measure of time.

A masculine noun in Old English. In Greek, Italic, Celtic, and Armenian the cognate words now mean only "month." Greek selēnē (Lesbian selanna) is from selas "light, brightness (of heavenly bodies)." Old Norse also had tungl "moon," ("replacing mani in prose" - Buck), evidently an older Germanic word for "heavenly body," cognate with Gothic tuggl, Old English tungol "heavenly body, constellation," of unknown origin or connection. Hence Old Norse tunglfylling "lunation," tunglœrr "lunatic" (adj.).

Extended 1665 to satellites of other planets. Typical of a place impossible to reach or a thing impossible to obtain, by 1590s. Meaning "a month, the period of the revolution of the moon about the earth" is from late 14c.

To shoot the moon "leave without paying rent" is British slang from c. 1823 (see shoot (v.)); the card-playing sense perhaps was influenced by gambler's shoot the works (1922) "go for broke" in shooting dice. The moon race and the U.S. space program of the 1960s inspired a number of coinages, including, from those skeptical of the benefits to be gained, moondoggle (based on boondoggle). The man in the moon "fancied semblance of a man seen in the disk of the full moon" is mentioned since early 14c.; he carries a bundle of thorn-twigs and is accompanied by a dog. Some Japanese, however, see a rice-cake-making rabbit in the moon. The old moon in the new moon's arms (1727) is the appearance of the moon in the first quarter, in which the whole orb is faintly visible by earthshine.

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moon (v.)

c. 1600, "to expose to moonlight;" later "idle about, wander or gaze moodily" (1836), "move listlessly" (1848), probably on the notion also found in moonstruck. The meaning "to flash the buttocks" is recorded by 1968, U.S. student slang, from moon (n.) "buttocks" (1756), "probably from the idea of pale circularity" [Ayto]. See moon (n.). Related: Mooned; mooning.

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moon-calf (n.)

also mooncalf, "abortive, shapeless, fleshy mass," 1560s, attributed to the influence of the moon; from moon (n.) + calf (n.1). In later 16c., "deformed creature, monster;" from 1620s as "congenital idiot."

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moor (v.)

"to fasten (a ship) in a particular location by or as by cables, anchors, etc.," late 15c., probably related to Old English mærels "mooring rope," via unrecorded *mærian "to moor," or possibly borrowed from Middle Low German moren or Middle Dutch maren "to moor," from West Germanic *mairojan. Related: Moored, mooring. French amarrer is from Dutch.

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moody (adj.)

"angry, quarrelsome," 12c., from Old English modig "brave, proud, high-spirited, impetuous, arrogant," from Proto-Germanic *modago- (source also of Old Saxon modag, Dutch moedig, German mutig, Old Norse moðugr); see mood (n.1) + -y (2). Meaning "subject to or indulging in gloomy spells, out of humor, sullen" is recorded by 1590s (via the Middle English sense of "angry"). Related: Moodily.

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moonstruck (adj.)

"affected in mind or health by the light of the moon; lunatic, crazed," 1670s, from moon (n.) + struck (see strike (v.)). Compare Greek selenobletos. For sense, see moon (v.). Perhaps coined by Milton ("Paradise Lost").

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moon-dog (n.)

dog who bays at the moon, 1660s, from moon (n.) + dog (n.). Earlier in same sense was mooner (1570s).

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moonglow (n.)

"the light or glow of the moon," 1926, from moon (n.) + glow (n.).

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