The Collaborative International Dictionary of English v.0.48:

Shell \Shell\, n. [OE. shelle, schelle, AS. scell, scyll; akin
   to D. shel, Icel. skel, Goth. skalja a tile, and E. skill.
   Cf. Scale of fishes, Shale, Skill.]
   1. A hard outside covering, as of a fruit or an animal.
      Specifically:
      (a) The covering, or outside part, of a nut; as, a
          hazelnut shell.
      (b) A pod.
      (c) The hard covering of an egg.
          [1913 Webster]

                Think him as a serpent's egg, . . .
                And kill him in the shell.        --Shak.
          [1913 Webster]
      (d) (Zool.) The hard calcareous or chitinous external
          covering of mollusks, crustaceans, and some other
          invertebrates. In some mollusks, as the cuttlefishes,
          it is internal, or concealed by the mantle. Also, the
          hard covering of some vertebrates, as the armadillo,
          the tortoise, and the like.
      (e) (Zool.) Hence, by extension, any mollusks having such
          a covering.
          [1913 Webster]

   2. (Mil.) A hollow projectile, of various shapes, adapted for
      a mortar or a cannon, and containing an explosive
      substance, ignited with a fuse or by percussion, by means
      of which the projectile is burst and its fragments
      scattered. See Bomb.
      [1913 Webster]

   3. The case which holds the powder, or charge of powder and
      shot, used with breechloading small arms.
      [1913 Webster]

   4. Any slight hollow structure; a framework, or exterior
      structure, regarded as not complete or filled in; as, the
      shell of a house.
      [1913 Webster]

   5. A coarse kind of coffin; also, a thin interior coffin
      inclosed in a more substantial one. --Knight.
      [1913 Webster]

   6. An instrument of music, as a lyre, -- the first lyre
      having been made, it is said, by drawing strings over a
      tortoise shell.
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            When Jubal struck the chorded shell.  --Dryden.
      [1913 Webster]

   7. An engraved copper roller used in print works.
      [1913 Webster]

   8. pl. The husks of cacao seeds, a decoction of which is
      often used as a substitute for chocolate, cocoa, etc.
      [1913 Webster]

   9. (Naut.) The outer frame or case of a block within which
      the sheaves revolve.
      [1913 Webster]

   10. A light boat the frame of which is covered with thin wood
       or with paper; as, a racing shell.
       [1913 Webster]

   11. Something similar in form or action to an ordnance shell;
       specif.:
       (a) (Fireworks) A case or cartridge containing a charge
           of explosive material, which bursts after having been
           thrown high into the air. It is often elevated
           through the agency of a larger firework in which it
           is contained.
       (b) (Oil Wells) A torpedo.
           [Webster 1913 Suppl.]

   12. A concave rough cast-iron tool in which a convex lens is
       ground to shape.
       [Webster 1913 Suppl.]

   13. A gouge bit or shell bit.
       [Webster 1913 Suppl.]

   Message shell, a bombshell inside of which papers may be
      put, in order to convey messages.

   Shell bit, a tool shaped like a gouge, used with a brace in
      boring wood. See Bit, n., 3.

   Shell button.
       (a) A button made of shell.
       (b) A hollow button made of two pieces, as of metal, one
           for the front and the other for the back, -- often
           covered with cloth, silk, etc.

   Shell cameo, a cameo cut in shell instead of stone.

   Shell flower. (Bot.) Same as Turtlehead.

   Shell gland. (Zool.)
       (a) A glandular organ in which the rudimentary shell is
           formed in embryonic mollusks.
       (b) A glandular organ which secretes the eggshells of
           various worms, crustacea, mollusks, etc.

   Shell gun, a cannon suitable for throwing shells.

   Shell ibis (Zool.), the openbill of India.

   Shell jacket, an undress military jacket.

   Shell lime, lime made by burning the shells of shellfish.


   Shell marl (Min.), a kind of marl characterized by an
      abundance of shells, or fragments of shells.

   Shell meat, food consisting of shellfish, or testaceous
      mollusks. --Fuller.

   Shell mound. See under Mound.

   Shell of a boiler, the exterior of a steam boiler, forming
      a case to contain the water and steam, often inclosing
      also flues and the furnace; the barrel of a cylindrical,
      or locomotive, boiler.

   Shell road, a road of which the surface or bed is made of
      shells, as oyster shells.

   Shell sand, minute fragments of shells constituting a
      considerable part of the seabeach in some places.
      [1913 Webster]

WordNet (r) 3.0 (2006):

shell jacket
    n 1: waist-length jacket tapering to a point at the back; worn
         by officers in the mess for formal dinners [syn: mess
         jacket, monkey jacket, shell jacket]