Young codfish

• Mine-roof prop
• Prop in a mine
• A chock or bar wedged under a wheel or between the spokes to prevent a vehicle from rolling down an incline
• A wooden prop used to support the roof of a mine
• Wagon prop
• Wedge placed under a wheel
• Wedge placed beneath a wheel

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.

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"prop in a mine," 1841, of unknown origin. Transferred by 1878 to wood blocks, etc., used to brake motor vehicles. As a verb, from 1841. Related: Spragged ; spragging .