The Collaborative International Dictionary of English v.0.48:

lockbox \lock"box`\, n.
   A box of strong and durable construction, fitted with a lock,
   used for the purpose of protecting valuable items, such as
   money or jewelry; a strongbox.
   [PJC]

WordNet (r) 3.0 (2006):

lockbox
    n 1: a fireproof metal strongbox (usually in a bank) for storing
         valuables [syn: safe-deposit, safe-deposit box,
         safety-deposit, safety deposit box, deposit box,
         lockbox]