Comment by frereubu

Interesting, although could those not be different things? The linked page - https://en.wikipedia.org/wiki/Grain_(unit) - shows that it absolutely was a foundational measure in England. I guess it's probably down to the etymology, which would tell you when and where the phrase came from, then you could tell whether it from the English or ancient meaning, although it's not especially relevant to the meaning. The linked article does call it an "English idiom", so you'd need to show a similar ancient idiom for that to be the source.