flat (adj.) Look up flat at Dictionary.com
early 14c., from Old Norse flatr, from Proto-Germanic *flataz (cognates: 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" (source of 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).
flat (n.) Look up flat at Dictionary.com
1801, from Scottish flat "floor or story of a house," from Old English flet "a dwelling, floor, ground," from the same source as flat (adj.).