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]