Is it ensure, insure, or assure?
1947, RAF slang for "navigational instruments;" later extended to any sort of apparatus that operates in a sealed container. Especially of flight recorders from c.1964.
A crash-resistant steel container that holds instruments that record performance data in airplanes. The data are used to analyze the causes of accidents.
Secrecy; classified material; intelligence interests: I suspected the project might involve secret military operations. ''I knew there was black box involved with it''/ A highly confidential annex, placed in a black box, contains selected ''raw material''
[1970s+; fr the use by British intelligence agencies of a black box for highly secret material for the prime minister and the defense and foreign secretaries]