Английский Этимологический словарь - flat (adj.)

- c.1300, from O.N. flatr, from P.Gmc. *flataz, from PIE *pla-. Sense of "prosaic, dull" is from 1573; used of drink from 1607; of musical notes from 1591. Flat-footed "unprepared" is from 1912, earlier "straightforwardly" (1828). Flat-out "open, direct," originally meant "total failure" (1870). Flatware is distinguished from hollow ware.