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.''

I called Tom Wolfe, whose current novel, ''The Bonfire of the Vanities,'' pushes the envelope of urban high life, to find out where he picked it up. ''I first heard it in 1972, among test pilots who later became astronauts. They were speaking of the performance capabilities of an airplane as an envelope, as if there were a boundary. Why they chose >envelope, I don't know, but if you get outside the envelope, you're in trouble.'' He estimated that its test-pilot use may have begun at the Patuxent River Naval Air Station in Maryland in the 1940's.

Let's push back further with the help of the Oxford English Dictionary Supplement. As an aeronautical term, >envelope was in use as early as 1901, meaning ''the gas or air container of a balloon or airship.'' In August of that year, Scientific American wrote: ''The balloon is inflated with hydrogen, and in order to maintain at all times a tension on the envelope -that is to say, perfect inflation - a compensating balloon filled with air is placed in the interior.''

In 1944, the Journal of the Royal Aeronautics Society extended that meaning of ''perfect inflation'' of a balloon to cover optimum flight performance of any kind of aircraft: ''Tests at other heights can then be confined to what are termed 'envelope' conditions; that is, the engine conditions which will give the maximum economy at any given speed.''

So there were those right-stuff test pilots, familiar with the phrase >envelope conditions, talking about ways to extend, change, enlarge or make more demanding those conditions - from the inside of an aircraft, to ''push'' the envelope beyond known borders into untested conditions. (Why the verb >push? Slang senses of >pushing it - as in ''You're pushing it, Buster'' - are ''extending beyond reason'' and ''pressing past boundaries of discretion.'') The space connection is unbroken; William Broyles Jr. wrote in U.S. News & World Report in 1986 of Christa McAuliffe, the teacher who was a member of the Challenger crew, ''pushing out the envelope of the planet.'' And the metaphorical envelope was also taken up by the military: ''The three-dimensional space,'' goes the definition in the Dictionary of Military Terms, ''that is within range, altitude and deflection reach of a weapon, particularly an air defense weapon.''