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Attested in "True Grit" by Charles Portis, 1968, but most likely much older. The derivation is probably from the percussion "cap," a small metal cylinder open at one end with an amount of shock-sensitive explosive, usually fulminate of mercury, used to set off the powder charge in a muzzle-loading firearm. The modern derivation, however, is probably from the "cap gun," a toy firearm using paper "caps" containing Armstrong's mixture or a similar substance to provide the small explosions.
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