blue John

• bluejohn, blue john, blue-john, blue-John, Blue John

From blue + John.

• (UK) IPA^(key): /ˈbluː dʒɒn/

blue John (uncountable)

1. (obsolete, brewing) Afterwort, partially fermented wort. [17th–19th c.]
2. A semiprecious mineral mined in Derbyshire, a form of fluorite with bands of a purple-blue or yellowish colour. [from 18th c.]
3. (US regional) Skimmed milk. [from 19th c.]
   • 1968, Charles Portis, True Grit:

     Lee went up front to his icebox and brought back a jar of milk. The cream had been skimmed from it. I said, “This tastes like blue-john to me.”

• sky blue