flatyoudaoicibaDictYouDict[flat 词源字典]
flat: [14] The Old English word for ‘flat’ was efen ‘even’, and flat was not acquired until Middle English times, from Old Norse flatr. This came from a prehistoric Germanic *flataz, source also of German platt ‘flat’. And *flataz probably goes back to an Indo-European *pelə -, *plā-, denoting ‘spread out flat’, from which came Sanskrit prthūs ‘broad’, Greek platūs ‘broad’ (source of English place, plaice, plane [the tree], and platypus), Latin plānus ‘flat’ (whence English plane and plain ‘unadorned’), and also English place, plaice, plant, and flan. Flat ‘single-storey dwelling’ [19] is ultimately the same word, but it has a more circuitous history.

It is an alteration (inspired no doubt by the adjective flat) of a now obsolete Scottish word flet ‘interior of a house’, which came from a prehistoric Germanic *flatjam ‘flat surface, floor’, a derivative of the same source (*flataz) as produced the adjective.

=> flan, flatter, floor, place, plaice, plane, platypus[flat etymology, flat origin, 英语词源]
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.