The Collaborative International Dictionary of English v.0.48:

Flat \Flat\ (fl[a^]t), a. [Compar. Flatter (fl[a^]t"r[~e]r);
   superl. Flattest (fl[a^]t"t[e^]st).] [Akin to Icel. flatr,
   Sw. flat, Dan. flad, OHG. flaz, and AS. flet floor, G.
   fl["o]tz stratum, layer.]
   1. Having an even and horizontal surface, or nearly so,
      without prominences or depressions; level without
      inclination; plane.
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            Though sun and moon
            Were in the flat sea sunk.            --Milton.
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   2. Lying at full length, or spread out, upon the ground;
      level with the ground or earth; prostrate; as, to lie flat
      on the ground; hence, fallen; laid low; ruined; destroyed.
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            What ruins kingdoms, and lays cities flat! --Milton.
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            I feel . . . my hopes all flat.       --Milton.
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   3. (Fine Arts) Wanting relief; destitute of variety; without
      points of prominence and striking interest.
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            A large part of the work is, to me, very flat.
                                                  --Coleridge.
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   4. Tasteless; stale; vapid; insipid; dead; as, fruit or drink
      flat to the taste.
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   5. Unanimated; dull; uninteresting; without point or spirit;
      monotonous; as, a flat speech or composition.
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            How weary, stale, flat, and unprofitable
            Seem to me all the uses of this world. --Shak.
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   6. Lacking liveliness of commercial exchange and dealings;
      depressed; dull; as, the market is flat.
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   7. Clear; unmistakable; peremptory; absolute; positive;
      downright.

   Syn: flat-out.
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              Flat burglary as ever was committed. --Shak.
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              A great tobacco taker too, -- that's flat.
                                                  --Marston.
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   8. (Mus.)
      (a) Below the true pitch; hence, as applied to intervals,
          minor, or lower by a half step; as, a flat seventh; A
          flat.
      (b) Not sharp or shrill; not acute; as, a flat sound.
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   9. (Phonetics) Sonant; vocal; -- applied to any one of the
      sonant or vocal consonants, as distinguished from a
      nonsonant (or sharp) consonant.
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   10. (Golf) Having a head at a very obtuse angle to the shaft;
       -- said of a club.
       [Webster 1913 Suppl.]

   11. (Gram.) Not having an inflectional ending or sign, as a
       noun used as an adjective, or an adjective as an adverb,
       without the addition of a formative suffix, or an
       infinitive without the sign to. Many flat adverbs, as in
       run fast, buy cheap, are from AS. adverbs in -["e], the
       loss of this ending having made them like the adjectives.
       Some having forms in ly, such as exceeding, wonderful,
       true, are now archaic.
       [Webster 1913 Suppl.]

   12. (Hort.) Flattening at the ends; -- said of certain
       fruits.
       [Webster 1913 Suppl.]

   Flat arch. (Arch.) See under Arch, n., 2. (b).

   Flat cap, cap paper, not folded. See under Paper.

   Flat chasing, in fine art metal working, a mode of
      ornamenting silverware, etc., producing figures by dots
      and lines made with a punching tool. --Knight.

   Flat chisel, a sculptor's chisel for smoothing.

   Flat file, a file wider than its thickness, and of
      rectangular section. See File.

   Flat nail, a small, sharp-pointed, wrought nail, with a
      flat, thin head, larger than a tack. --Knight.

   Flat paper, paper which has not been folded.

   Flat rail, a railroad rail consisting of a simple flat bar
      spiked to a longitudinal sleeper.

   Flat rods (Mining), horizontal or inclined connecting rods,
      for transmitting motion to pump rods at a distance.
      --Raymond.

   Flat rope, a rope made by plaiting instead of twisting;
      gasket; sennit.

   Note: Some flat hoisting ropes, as for mining shafts, are
         made by sewing together a number of ropes, making a
         wide, flat band. --Knight.

   Flat space. (Geom.) See Euclidian space.

   Flat stitch, the process of wood engraving. [Obs.] -- Flat
   tint (Painting), a coat of water color of one uniform shade.


   To fall flat (Fig.), to produce no effect; to fail in the
      intended effect; as, his speech fell flat.
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            Of all who fell by saber or by shot,
            Not one fell half so flat as Walter Scott. --Lord
                                                  Erskine.
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The Collaborative International Dictionary of English v.0.48:

Cap \Cap\ (k[a^]p), n. [OE. cappe, AS. c[ae]ppe, cap, cape,
   hood, fr. LL, cappa, capa; perhaps of Iberian origin, as
   Isidorus of Seville mentions it first: "Capa, quia quasi
   totum capiat hominem; it. capitis ornamentum." See 3d Cape,
   and cf. 1st Cope.]
   1. A covering for the head; esp.
      (a) One usually with a visor but without a brim, for men
          and boys;
      (b) One of lace, muslin, etc., for women, or infants;
      (c) One used as the mark or ensign of some rank, office,
          or dignity, as that of a cardinal.
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   2. The top, or uppermost part; the chief.
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            Thou art the cap of all the fools alive. --Shak.
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   3. A respectful uncovering of the head.
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            He that will give a cap and make a leg in thanks.
                                                  --Fuller.
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   4. (Zool.) The whole top of the head of a bird from the base
      of the bill to the nape of the neck.
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   5. Anything resembling a cap in form, position, or use; as:
      (a) (Arch.) The uppermost of any assemblage of parts; as,
          the cap of column, door, etc.; a capital, coping,
          cornice, lintel, or plate.
      (b) Something covering the top or end of a thing for
          protection or ornament.
      (c) (Naut.) A collar of iron or wood used in joining
          spars, as the mast and the topmast, the bowsprit and
          the jib boom; also, a covering of tarred canvas at the
          end of a rope.
      (d) A percussion cap. See under Percussion.
      (e) (Mech.) The removable cover of a journal box.
      (f) (Geom.) A portion of a spherical or other convex
          surface.
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   6. A large size of writing paper; as, flat cap; foolscap;
      legal cap.
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   Cap of a cannon, a piece of lead laid over the vent to keep
      the priming dry; -- now called an apron.

   Cap in hand, obsequiously; submissively.

   Cap of liberty. See Liberty cap, under Liberty.

   Cap of maintenance, a cap of state carried before the kings
      of England at the coronation. It is also carried before
      the mayors of some cities.

   Cap money, money collected in a cap for the huntsman at the
      death of the fox.

   Cap paper.
      (a) A kind of writing paper including flat cap, foolscap,
          and legal cap.
      (b) A coarse wrapping paper used for making caps to hold
          commodities.

   Cap rock (Mining), The layer of rock next overlying ore,
      generally of barren vein material.

   Flat cap, cap See Foolscap.

   Forage cap, the cloth undress head covering of an officer
      of soldier.

   Legal cap, a kind of folio writing paper, made for the use
      of lawyers, in long narrow sheets which have the fold at
      the top or "narrow edge."

   To set one's cap, to make a fool of one. (Obs.) --Chaucer.

   To set one's cap for, to try to win the favor of a man with
      a view to marriage. [Colloq.]
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WordNet (r) 3.0 (2006):

flat cap
    n 1: a flat woolen cap with a stiff peak [syn: cloth cap,
         flat cap]