The Collaborative International Dictionary of English v.0.48:

Sprag \Sprag\, a.
   See Sprack, a. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sprag \Sprag\ (spr[a^]g), n. [Cf. Icel. spraka a small
   flounder.] (Zool.)
   A young salmon. [Prov. Eng.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sprag \Sprag\, n. [See Spray a branch.]
   A billet of wood; a piece of timber used as a prop.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sprag \Sprag\, v. t. [imp. & p. p. Spragged (spr[a^]gd); p.
   pr. & vb. n. Spragging (-g[i^]ng).]
   1. To check the motion of, as a carriage on a steep grade, by
      putting a sprag between the spokes of the wheel. --R. S.
      Poole.
      [1913 Webster]

   2. To prop or sustain with a sprag.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sprag
    n 1: a chock or bar wedged under a wheel or between the spokes
         to prevent a vehicle from rolling down an incline
    2: a wooden prop used to support the roof of a mine [syn:
       pitprop, sprag]