I'd never heard of this before, but just found the following on a truly excellent site.....


"Push the envelope" is now used figuratively to mean "stretch the
boundaries". (The image is not of pushing a mailing envelope across
a desk: those who push this sort of envelope do it from within. Cf.
"pressing the limits".) On its AOL message board, Merriam-Webster
Editorial Department writes: "A sentence we spotted in a 1991 issue
of the Wall Street Journal provides a typical example of the use of
the phrase [...]: 'Ads...seem to be pushing the envelope of taste
every day.' 'Push the envelope' in this sense is a very recent
arrival on the scene, dating only from 1988 according to the
evidence in our files.

"The phrase has its origins in the world of aviation, where
'envelope' has, since at least the late 60s, had the meaning 'a set
of performance limits that may not be safely exceeded.' Test pilots
are often called on to 'push' a new aircraft's performance envelope
by going beyond known safety limits, as in determining just how fast
an airplane can be flown. In 1979 Tom Wolfe's best-seller 'The Right
Stuff' vividly described the life of test pilots during the 50s and
60s, and it appears that this book, and the subsequent movie, did
much to popularize the notion of pushing the envelope. [Stuart
Leichter reports that the words used in the movie are "pushing the
outside of the envelope"; someone should check what they were in the
book.]

"The idea of an envelope as a kind of enclosing boundary is of
course not new. In 1899 Arnold Bennett wrote: 'My desire is to
depict the deeper beauty while abiding by the envelope of facts.'"