"beam of light, light emitted in a given direction from a luminous body," early 14c., rai, from Old French rai (nominative rais) "ray (of the sun), spoke (of a wheel); gush, spurt," from Latin radius "ray, spoke, staff, rod" (see radius). Not common before 17c. [OED]; of the sun, usually in reference to heat (beam being preferred for light).
Ray is usually distinguished from beam, as indicating a smaller amount of light; in scientific use a beam is a collection of parallel rays. In ordinary language ray is the word usually employed when the reference is to the heat rather than the light of the sun .... [OED]
Science fiction's ray-gun is recorded by 1931 (in Amazing Stories; electric ray gun as an imaginary weapon is from 1924; death-ray gun from 1926 as a prop in a vaudeville act), but the Martians had a Heat-Ray weapon in "War of the Worlds" (1898).
"a skate, type of fish related to sharks and noted for its broad, flat body," early 14c., raie, from Old French raie (13c.) and directly from Latin raia. De Vaan describes this as a word of unknown origin but with apparent cognates in Germanic (Middle Dutch rogghe, Old English reohhe), perhaps a loan-word from a substrate language. The old etymology (Century Dictionary, etc.) was that the fish was so called from its resemblance to the rays of a fan and from the source of ray (n.1).
"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.
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.
"a long time," 1821, often in phrases indicating something rarely occurring. Compare at the Greek calends (from an ancient Roman phrase alluding to the fact that the Greeks had nothing corresponding to the Roman calends), and the native in the reign of Queen Dick and Saint Geoffrey's Day "Never, there being no saint of that name," reported in Grose (1788). Nevermass "date which never comes" is from 1540s. Blue moon is suggested earliest in this couplet from 1528:
Yf they say the mone is blewe,
We must beleve that it is true.
Though this might refer to calendrical calculations by the Church. Thus the general "rareness" sense of the term is difficult to disentangle from the specific calendrical one (commonly misinterpreted as "second full moon in a calendar month," but actually a quarterly calculation). In either case, the sense of blue here is obscure. Literal blue moons do sometimes occur under extreme atmospheric conditions.