The Collaborative International Dictionary of English v.0.48:

Holster \Hol"ster\, n. [D. holster; skin to AS. heolstor den,
   cave, fr. helan to conceal, and to Icel. hulstr case, Goth.
   hulistr covering, veil, huljan to cover. [root]17. See Hele
   to cover, Hell, and cf. Housing, Houss.]
   A leather case for a pistol, carried by a horseman at the bow
   of his saddle, or worn on the person suspended from a belt or
   shoulder strap.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

holster
    n 1: a sheath (usually leather) for carrying a handgun
    2: a belt with loops or slots for carrying small hand tools