1848. Lowell, Fable for Critics, p. 19. (A fetch, I must say, most transparent and flat).
[There are other usages, more or less colloquial: e.g., Insipid; tame; dull: as in Macaulay’s “flat as champagne in decanters.” On the Stock Exchange. flat = without interest: Stock is borrowed flat when no interest is allowed by the lender as security for the due return of the scrip.] [[15]]
Verb (American). To jilt. Cf., subs., sense 3. For synonyms, see Mitten.
1871. De Vere, Americanisms, p. 602. To flat, in the West, means to jilt, and is probably derived from another slang phrase, ‘to feel flat,’ denoting the depression which is apt to follow such a disappointment.
To feel flat, verb. phr. (American).—1. To be low-spirited; out of sorts; off colour (q.v.).
1838. J. C. Neal, Charcoal Sketches. Not to hurt a gentleman’s feelings and to make him feel flat afore the country.
2. (American).—To fail; to give way. Also used substantively.
Flat as a flounder (or Pancake), phr. (colloquial).—Very flat indeed. Also flat as be blowed.
1882. Punch, vol. LXXXII., p. 177, col. 1.
To brush up a flat. See Brusher.