One who is not sharp; a suite of rooms on one floor.
“Oh, Messrs ... what flats you are!” —The Times.
“He said he was going to have a flat to let on the top floor.” — Howells: Hazards of New Fortunes. vol. i. part i. p. 123.
Flat as a flounder. I knocked him down flat as a flounder. A flounder is one of the flat-fish. Flat as a pancake. Quite flat. A pancake is a thin flat cake, fried in a pan.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894