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.
n.
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).
adv.
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).
v.
c.1600, "to lay flat;" 1670s in music, from flat, adj.. Related: Flatted; flatting.
词根：flat = to blow（吹）
来自拉丁语的flat 意为to blow。
（*拉丁文flare(=to blow),过去分词是flatus。）
同源词：
1. inflate（在里面吹 in(=in) + flat(=blow)） vt.使（通货）膨胀，提高物价；使充气；使得意 inflation n.膨胀；通货膨胀 inflatable adj.可充气的；可膨胀的 inflationary adj.通货膨胀的 The street vendors often inflate their prices for tourists .路边的小贩常常向游人的高价格。
2. deflate v.紧缩（通货）；放出（车胎等的）气体，抽出空气；使某人泄气 *de(=down) deflate a pompous politician 使傲慢的政治家骄气大挫
3. deflationary adj.通货紧缩的 *de(=down) deflationary measures applied by the Secretary of Treasury. 由财政部长实施的通货紧缩措施