Is it ensure, insure, or assure?
"leather case for a pistol," 1660s, probably from Old English heolster, earlier helustr "concealment, hiding place," from Proto-Germanic *hulfti- (cf. Old High German hulft "cover, case, sheath," Old Norse hulstr "case, sheath," Middle Dutch holster, German Holfster "holster"), from PIE *kel- "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.