Old English gad "point, spearhead, arrowhead, pointed stick used for driving cattle," from Proto-Germanic *gaido "goad, spear" (source also of Lombardic gaida "spear"), which is perhaps cognate with Sanskrit hetih "missile, projectile," himsati "he injures;" Avestan zaena- "weapon;" Greek khaios "shepherd's staff;" Old English gar "spear;" Old Irish gae "spear." Figurative use "anything that urges or stimulates" is since 16c., probably from the Bible.
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