Here's an excerpt from the New York Times about it:

The phrase was popularized in 1979 by Tom Wolfe in his book about astronauts, ''The Right Stuff.'' The phrase-sensitive reporter wrote: ''The 'envelope' was a flight-test term referring to the limits of a particular aircraft's performance, how tight a turn it could make at such-and-such a speed, and so on. 'Pushing the outside,' probing the outer limits, of the envelope seemed to be the great challenge and satisfaction of flight test.''

Source:

https://www.nytimes.com/1988/05/15/magazine/on-language-pushing-the-envelope.html

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