Etymology
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sprag (n.)

"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.

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Definitions of sprag

sprag (n.)
a chock or bar wedged under a wheel or between the spokes to prevent a vehicle from rolling down an incline;
sprag (n.)
a wooden prop used to support the roof of a mine;
Synonyms: pitprop
From wordnet.princeton.edu