holster: [17] Holster was probably borrowed from Dutch holster, and may well be related to Old English heolster ‘cover’, which did not survive into the Middle English period. It seems likely that its ultimate source was Indo- European *kel- ‘cover, hide’, a prolific progenitor of English words including apocalypse, cell, cellar, conceal, hall, hell, helmet, hole, hollow, hull ‘pod’, and occult. => apocalypse, cell, conceal, hall, hell, helmet, occult
holster (n.)
"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.
holster (v.)
by 1902, from holster (n.). Related: Holstered; holstering.