Source : Webster's Revised Unabridged Dictionary (1913)

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.

            The dead knight's sword out of his sheath he drew.
                                                  --Spenser.

   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) (Zo["o]l.) One of the elytra of an insect.

   {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}.

Source : WordNet®

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