Douglas Harper's Etymology Dictionary
Usage examples of "strong-box".
Bligh striking out the debts from her ledger-book, her strong-box becoming a catch-basin for the new money, overflowing and spilling out gleaming rivulets down the street to the bankside coffee-merchants, and thence down the Thames into the wide.
Christopher Brockbank always deposited or withdrew documents from that strong-box in the presence of an official of the bank who was in a position to identify him beyond question.
They've got the measure of these gravies now, where they build their strong-boxes.