"leather case for a pistol," 1660s, probably from Old English heolster, earlier helustr "concealment, hiding place," from Proto-Germanic *hulfti- (cognates: Old High German hulft "cover, case, sheath," Old Norse hulstr "case, sheath," Middle Dutch holster, German Halfter "holster"), from PIE *kel- (2) "to cover, to hide" (see cell). Intermediate forms are wanting, and the modern word could as well be from the Norse or Dutch cognates.

by 1902, from holster, n.. Related: Holstered; holstering.