Wedge placed under a wheel

• Roof prop in a coal mine
• A chock or bar wedged under a wheel or between the spokes to prevent a vehicle from rolling down an incline
• Wedge stuck under a wheel
• A wooden prop used to support the roof of a mine
• Timber prop
• Piece serving as a prop or brake

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sprag \Sprag\, n. [See Spray a branch.] A billet of wood; a piece of timber used as a prop.

Wiktionary Word definitions in Wiktionary
Etymology 1 (alternative form of sprack English) Etymology 2 n. A billet of wood; a piece of timber, a similar solid object or constructed unit used as a prop. v 1 (context transitive English) To check the motion of, as a carriage on a steep slope, by putting ...