flasher (n.)youdaoicibaDictYouDict[flasher 词源字典]
1680s, "something that emits light in flashes," agent noun from flash (v.). Meaning "male genital exhibitionist" is from 1960s (meat-flasher in this sense was attested in 1890s and flash (v.) in the sense "expose the genitals" is recorded by 1846). Johnson (1755) has it also in the sense "one who makes a show of more wit than he possesses."[flasher etymology, flasher origin, 英语词源]
flashing (adj.)youdaoicibaDictYouDict
1540s, of light; present participle adjective from flash (v.).
flashing (n.)youdaoicibaDictYouDict
1791, "act of creating an artificial flood," verbal noun from flash (v.); also compare flash (n.2)). Meaning "indecent exposure" is by 1968 (see flasher). The meaning "strip of metal used in roofing, etc." is from 1782, earlier simply flash (1570s), but the sense connection is unclear and it might be an unrelated word.
flashlight (n.)youdaoicibaDictYouDict
also flash-light, 1886, "on-and-off signal light in a light-house, etc.," from flash (v.) + light (n.). As the word for a photographer's light-emitting preparation, 1892 (flash-lamp in this sense is by 1890). From 1905 as as a handheld, pocket-sized electric illumination device, the American English word for what the British might call an electric torch.
flashy (adj.)youdaoicibaDictYouDict
"showy, cheaply attractive," 1680s, from flash (n.1) + -y (2). Earlier it meant "splashing" (1580s); "sparkling, giving off flashes" (c. 1600), but those senses have become rare. Related: Flashily; flashiness.
flask (n.)youdaoicibaDictYouDict
mid-14c., from Medieval Latin flasco "container, bottle," from Late Latin flasconem (nominative flasco) "bottle," which is of uncertain origin. A word common to Germanic and Romanic, but it is unclear whether the Latin or Germanic word is the original (or whether both might have got it from the Celts). Those who support a Germanic origin compare Old English flasce "flask, bottle" (which would have become modern English *flash), Old High German flaska, Middle Dutch flasce, German Flasche "bottle." If it is Germanic, the original sense might be "bottle plaited round, case bottle" (compare Old High German flechtan "to weave," Old English fleohtan "to braid, plait"), from Proto-Germanic base *fleh- (see flax).

Another theory traces the Late Latin word to a metathesis of Latin vasculum. "The assumption that the word is of Teut[onic] origin is chronologically legitimate, and presents no difficulty exc[ept] the absence of any satisfactory etymology" [OED]. The similar words in Finnish and Slavic are held to be from Germanic.
flat (adj.)youdaoicibaDictYouDict
c. 1300, "stretched out (on a surface), prostrate, lying the whole length on the ground;" mid-14c., "level, all in one plane; even, smooth;" of a roof, "low-pitched," from Old Norse flatr "flat," from Proto-Germanic *flata- (cognates: Old Saxon flat "flat, shallow," Old High German flaz "flat, level," Old English flet (for which see flat (n.)), Old High German flezzi "floor"), from PIE *plat- "to spread" (source of Greek platys "broad, flat;" see plaice (n.)). From c. 1400 as "without curvature or projection."

Sense of "prosaic, dull" is from 1570s, on the notion of "featureless, lacking contrast." Used of drink from c. 1600; of musical notes from 1590s, because the tone is "lower" than a given or intended pitch; of women's bosoms by 1864. Flat tire or flat tyre is from 1908. Flat-screen (adj.) in reference to television is from 1969 as a potential technology. Flat-earth (adj.) in reference to refusal to accept evidence of a global earth, is from 1876.
flat (n.)youdaoicibaDictYouDict
1801, "a story of a house," from Scottish flat "floor or story of a house," from Old English flett "a dwelling; floor, ground," from the same source as flat (adj.). Meaning "floor or part of a floor set up as an apartment" is from 1824. Directly from flat (adj.) come the senses "level ground near water" (late 13c.); "a flat surface, the flat part of anything" (1374), and "low shoe" (1834).
flat (adv.)youdaoicibaDictYouDict
1550s, "absolutely, downright;" 1570s, "plainly, positively," from flat (adj.). Flat-out (adv.) "openly, directly" is from 1932, originally in motor racing, picked up in World War II by the airmen; earlier it was a noun meaning "total failure" (1870, U.S. colloquial).
flat (v.)youdaoicibaDictYouDict
c. 1600, "to lay flat;" 1670s in music, from flat (adj.). Related: Flatted; flatting.
flat-boat (n.)youdaoicibaDictYouDict
also flat-boat, 1650s, from flat (adj.) + boat (n.).
flat-car (n.)youdaoicibaDictYouDict
1839 in railroading, from flat (adj.) + car (n.).
flat-footed (adj.)youdaoicibaDictYouDict
c. 1600, "with flat feet;" see flat (adj.) + foot (n.). Meaning "unprepared" is from 1912, U.S. baseball slang, on notion of "not on one's toes;" earlier in U.S. colloquial adverbial use it meant "straightforwardly, downright, resolute" (1828), from notion of "standing firmly."
flat-iron (n.)youdaoicibaDictYouDict
"iron for smoothing," 1810, from flat (adj.) + iron (n.). Applied to triangular or wedge-shaped buildings from 1862.
flat-top (n.)youdaoicibaDictYouDict
1943, "aircraft carrier," U.S. Navy, from flat (adj.) + top (n.). As a style of haircut, from 1956.
flatfish (n.)youdaoicibaDictYouDict
also flat-fish, 1710, from flat (adj.) + fish (n.). So called from the shape.
flatland (n.)youdaoicibaDictYouDict
1735, from flat (adj.) + land (n.). Edwin Abbott's popular book about an imaginary two-dimensional world was published in 1884.
flatline (v.)youdaoicibaDictYouDict
"give no indication of life, cease to function," by 1998, from the flat (adj.) line (n.) on an electrocardiogram or electroencephalogram when the patient is dead. Related: Flatlined; flatlining.
flatly (adv.)youdaoicibaDictYouDict
early 15c. in a literal sense, from flat (adj.) + -ly (2). Meaning "in a plain manner" is from 1560s; sense of "in a dull manner" is from 1640s.
flatness (n.)youdaoicibaDictYouDict
mid-15c., "state or quality of being flat," from flat (adj.) + -ness.