Etymology
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X-ray (n.)
1896, X-rays, translation of German X-strahlen, from X, algebraic symbol for an unknown quantity, + Strahl (plural Strahlen) "beam, ray." Coined 1895 by German scientist Wilhelm Conrad Röntgen (1845-1923), who discovered them, to suggest that the exact nature of the rays was unknown. As a verb by 1899. Meaning "image made using X-rays" is from 1934, earlier in this sense was X-radiograph (1899).
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X 

The entire entry for X in Johnson's dictionary (1756) is: "X is a letter, which, though found in Saxon words, begins no word in the English language." Most English words beginning in -x- are of Greek origin (see chi) or modern commercial coinages. East Anglian in 14c. showed a tendency to use -x- for initial sh-, sch- (such as xal for shall), which didn't catch on but seems an improvement over the current system. As a symbol of a kiss on a letter, etc., it is recorded from 1765. In malt liquor, XX denoted "double quality" and XXX "strongest quality" (1827).

Algebraic meaning "unknown quantity" (1660 in English, from French), sometimes is said to be from medieval use, originally a crossed -r-, in that case probably from Latin radix (see root (n.)). Other theories trace it to Arabic (Klein), but a more prosaic explanation says Descartes (1637) took x, y, z, the last three letters of the alphabet, for unknowns to correspond to a, b, c, used for known quantities.

Used allusively for "unknown person" from 1797, "something unknown" since 1859. As a type of chromosome, attested from 1902 (first so called in German; Henking, 1891). To designate "films deemed suitable for adults only," first used 1950 in Britain; adopted in U.S. Nov. 1, 1968. The XYZ Affair in American history (1797) involved French agents designated by those letters.

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x (v.)
"cross out with an 'X'," 1942, from X.
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ray (n.1)

"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).

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ray (n.2)

"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).

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sting-ray (n.)
also sting ray, 1620s, from sting + ray (n.2). First in Capt. John Smith's writings: "Stingraies, whose tailes are very dangerous ...."
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mammogram (n.)

"X-ray image of the breast," by 1937, from mammo- "breast" + -gram.

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angiogram (n.)

"X-ray image of the arteries, veins, and/or heart chambers," 1933, from angio- + -gram.

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rad (n.)

1918, "x-ray dose unit," a shortened form of radiation (q.v.). The meaning "unit of absorbed dose of ionizing radiation" is by 1954, an acronym from radiation absorbed dose. As shortened form of radical (n.), it is attested in political slang from 1820. Teen slang adjectival sense of "extraordinary, wonderful" is from late 1970s (see radical (adj.)).

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chi (n.)
22nd letter of the Greek alphabet, representing a -kh- sound (see ch). The letter is shaped like an X, and so the Greek letter name was used figuratively to signify such a shape or arrangement (as in khiasma "two things placed crosswise;" khiastos "arranged diagonally; marked with an X;" khiazein "to mark with an 'X', to write the letter 'X'"). Some dialects used chi to represent the -ks- sound properly belonging to xi; Latin picked this up and the sound value of chi in Latin-derived alphabets is now that of English X.
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