The Collaborative International Dictionary of English v.0.48:

Sheath \Sheath\, n. [OE. schethe, AS. sc[=ae][eth],
   sce['a][eth], sc[=e][eth]; akin to OS. sk[=e][eth]ia, D.
   scheede, G. scheide, OHG. sceida, Sw. skida, Dan. skede,
   Icel. skei[eth]ir, pl., and to E. shed, v.t., originally
   meaning, to separate, to part. See Shed.]
   1. A case for the reception of a sword, hunting knife, or
      other long and slender instrument; a scabbard.
      [1913 Webster]

            The dead knight's sword out of his sheath he drew.
                                                  --Spenser.
      [1913 Webster]

   2. Any sheathlike covering, organ, or part. Specifically:
      (a) (Bot.) The base of a leaf when sheathing or investing
          a stem or branch, as in grasses.
      (b) (Zool.) One of the elytra of an insect.
          [1913 Webster]

   Medullary sheath. (Anat.) See under Medullary.

   Primitive sheath. (Anat.) See Neurilemma.

   Sheath knife, a knife with a fixed blade, carried in a
      sheath.

   Sheath of Schwann. (Anat.) See Schwann's sheath.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sheath
    n 1: a protective covering (as for a knife or sword)
    2: an enveloping structure or covering enclosing an animal or
       plant organ or part [syn: sheath, case]
    3: a dress suitable for formal occasions [syn: cocktail dress,
       sheath]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "sheath":
   Leatherette, Leatheroid, case, casing, coat, cuticle, dermis,
   encasement, fell, fleece, flesh, fur, furring, hide, imitation fur,
   imitation leather, integument, jacket, leather, leather paper,
   outer layer, outer skin, pelt, peltry, rawhide, rind, sheathing,
   skin, skins, tegument, vair