• If you drill down into the earlier results for the "box standard" Ngram, that looks like a spectacular fail from their (notoriously dodgy) data set.

    No idea about pushing the envelope, tho. Sounds aviation-based to me, no?

    @jj72 Aye. Even there, some people say it refers to the concept of a "Flight envelope", which is the general flight characteristics of an aeroplane that might be pushed by extreme use, but there's also the fact that the cone of high-pressure air that forms around a plane approaching supersonic speed is often referred to as an envelope, which makes "pushing the envelope" a more dramatic metaphor.

About

Avatar for   started