Adj. (colloquial and literary).—Downright; plain; straightforward; as
in that’s flat? a flat lie, “flat burglary,” etc.
1598. Shakspeare, 1 King Henry IV., Act I., Sc. 3. Wor.: You start
away, And lend no ear to my purposes. Those prisoners you shall
keep. Hot.: Nay, I will; that’s flat.
1835–40. Haliburton, The Clockmaker, p. 6, preface (ed. 1862).
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.]
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.