Why do we say "Pushing the envelope"?

Meaning:

Pushing any type of boundary.

Background:

This phrase comes from the aeronautical industry where the envelope refers to the safe performance limits of an aircraft - the envelope is the collection of curves that describe the maximum performance of an aircraft.

Apparently the expression was popularized by Tom Wolfe's 1979 book The Right Stuff, and the subsequent 1983 film adaptation, which recounts the story of the first seven astronauts selected for the NASA space program.

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