What's the origin of the phrase 'Push the envelope'?

This phrase came into general use following the publication Tom Wolfe's book about the space programme - The Right Stuff, 1979:

"One of the phrases that kept running through the conversation was ‘pushing the outside of the envelope’... [That] seemed to be the great challenge and satisfaction of flight test."

Wolfe didn't originate the term, although it's appropriate that he used it in a technical and engineering context, as it was first used in the field of mathematics.

The envelope here isn't the container for letters, but the mathematical envelope, which is defined as 'the locus of the ultimate intersections of consecutive curves'. In a two-dimensional example, the set of lines described by the various positions of a ladder sliding down a wall forms an envelope - in this case an arc, gently curving away from the intersection of the wall and floor. Inside that envelope you will be hit by the ladder; outside you won't.

So, blah blah this. So, blah blah that. This starting sentences with "So" has somehow crept into modern dialect and quite frankly I find it somewhat aggravating.

kincsem 02 Mar 20 22:22 
"the likes of" is the crutch of the trendy horse racing journalist.
The Derby was won by the likes of Shergar and Dancing Brave.

Gus Elen used that phrase in music-halls over 100 years ago. George Le Brunn, who also wrote for Marie Lloyd, wrote the music for this for him:

IT'S A GREAT BIG SHAME

They hadn't been married for a month or more
When underneath her thumb goes Jim
Oh isn't it a pity that the likes of her
Should put upon the likes of him.

“Look, I don’t like the tone of your voice”