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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