early 14c., from Old Norse flatr, from Proto-Germanic *flataz (cf. Old Saxon flat "flat, shallow,: Old High German flaz "flat, level," Old English flet, Old High German flezzi "floor"), perhaps from PIE *plat- "to spread" (cf. Greek platys "broad, flat;" see plaice (n.)).
Sense of "prosaic, dull" is from 1570s; used of drink from c.1600; of musical notes from 1590s, because the tone is "lowered." Flat-out (adv.) "openly, directly" is from 1932; earlier it was a noun meaning "total failure" (1870, U.S. colloquial).