The Collaborative International Dictionary of English v.0.48:

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

   Syn: lockbox.
        [PJC]

WordNet (r) 3.0 (2006):

strongbox
    n 1: a strongly made box for holding money or valuables; can be
         locked [syn: strongbox, deedbox]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "strongbox":
   Fort Knox, bank, bursary, cash register, cashbox, chest, coffer,
   coin box, depository, exchequer, fisc, gold depository, locker,
   money chest, penny bank, piggy bank, pork barrel, public crib,
   public till, public treasury, public trough, repository, safe,
   safe-deposit box, storehouse, strong room, subtreasury, till,
   treasure-house, treasury, vault