The Collaborative International Dictionary of English v.0.48:

Sheep \Sheep\, n. sing. & pl. [OE. shep, scheep, AS. sc?p,
   sce['a]p; akin to OFries. sk?p, LG. & D. schaap, G. schaf,
   OHG. sc[=a]f, Skr. ch[=a]ga. [root]295. Cf. Sheepherd.]
   1. (Zool.) Any one of several species of ruminants of the
      genus Ovis, native of the higher mountains of both
      hemispheres, but most numerous in Asia.
      [1913 Webster]

   Note: The domestic sheep (Ovis aries) varies much in size,
         in the length and texture of its wool, the form and
         size of its horns, the length of its tail, etc. It was
         domesticated in prehistoric ages, and many distinct
         breeds have been produced; as the merinos, celebrated
         for their fine wool; the Cretan sheep, noted for their
         long horns; the fat-tailed, or Turkish, sheep,
         remarkable for the size and fatness of the tail, which
         often has to be supported on trucks; the Southdowns, in
         which the horns are lacking; and an Asiatic breed which
         always has four horns.
         [1913 Webster]

   2. A weak, bashful, silly fellow. --Ainsworth.
      [1913 Webster]

   3. pl. Fig.: The people of God, as being under the government
      and protection of Christ, the great Shepherd.
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   Rocky mountain sheep.(Zool.) See Bighorn.

   Maned sheep. (Zool.) See Aoudad.

   Sheep bot (Zool.), the larva of the sheep botfly. See
      Estrus.

   Sheep dog (Zool.), a shepherd dog, or collie.

   Sheep laurel (Bot.), a small North American shrub (Kalmia
      angustifolia) with deep rose-colored flowers in corymbs.


   Sheep pest (Bot.), an Australian plant (Acaena ovina)
      related to the burnet. The fruit is covered with barbed
      spines, by which it adheres to the wool of sheep.

   Sheep run, an extensive tract of country where sheep range
      and graze.

   Sheep's beard (Bot.), a cichoraceous herb (Urospermum
      Dalechampii) of Southern Europe; -- so called from the
      conspicuous pappus of the achenes.

   Sheep's bit (Bot.), a European herb (Jasione montana)
      having much the appearance of scabious.

   Sheep pox (Med.), a contagious disease of sheep,
      characterixed by the development of vesicles or pocks upon
      the skin.

   Sheep scabious. (Bot.) Same as Sheep's bit.

   Sheep shears, shears in which the blades form the two ends
      of a steel bow, by the elasticity of which they open as
      often as pressed together by the hand in cutting; -- so
      called because used to cut off the wool of sheep.

   Sheep sorrel. (Bot.), a prerennial herb (Rumex
      Acetosella) growing naturally on poor, dry, gravelly
      soil. Its leaves have a pleasant acid taste like sorrel.


   Sheep's-wool (Zool.), the highest grade of Florida
      commercial sponges (Spongia equina, variety gossypina).


   Sheep tick (Zool.), a wingless parasitic insect
      (Melophagus ovinus) belonging to the Diptera. It fixes
      its proboscis in the skin of the sheep and sucks the
      blood, leaving a swelling. Called also sheep pest, and
      sheep louse.

   Sheep walk, a pasture for sheep; a sheep run.

   Wild sheep. (Zool.) See Argali, Mouflon, and Oorial.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sheep dog
    n 1: any of various usually long-haired breeds of dog reared to
         herd and guard sheep [syn: shepherd dog, sheepdog,
         sheep dog]