Mid 16th century (in the sense 'wrapper, enveloping layer'; originally as envelope): from French enveloppe, from envelopper 'envelop'. The sense 'covering of a letter' dates from the early 18th century.

• An envelope was originally any kind of wrapper or covering, not just something to put a letter in. It is from the same word as envelop, ‘to wrap up or surround’, from Old French envoluper the en- meaning ‘in’, but the origin of the rest is lost. To push the envelope is to go up to, or beyond, the limits of what is possible, an idea that comes from aeronautics. Since the Second World War the envelope or flight envelope has been the set of combinations of speed, altitude, and range within which a particular kind of aircraft can fly safely. If a test pilot is pushing the envelope he is flying the plane at the very limits of its performance. The phrase came into wider circulation after 1979 following its use in The Right Stuff, a book by American author Tom Wolfe about the early days of the American space programme, later made into an Oscar-winning film.