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]