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

SPRAG is a 5 letter word that starts with S.

1. \Sprag\, n. [Cf. Icel. spraka a small flounder.]
   (Zo["o]l.)
   A young salmon. [Prov. Eng.]
   
   

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

3. \Sprag\, v. t. [imp. & p. p. {Spragged}; p. pr. & vb. n.
   {Spragging}.]
   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.
   
   2. To prop or sustain with a sprag.
   
   

4. \Sprag\, a.
   See {Sprack}, a. --Shak.