(An). An apple so called from its being at maturity about St. John's Day (May 6th). We are told that apple-johns will keep for two years, and are best when shrivelled.
“I am withered like an old apple-john.” Shakespeare: 1 Henry IV. iii. 3.
Sometimes called the Apples of King John, which, if correct, would militate against the notion about “St. John's Day.”
“There were some things, for instance, the Apples of King John, ... I should be tempted to buy.” —Bigelow: Life of B. Franklin.
In the United States there is a drink called “Apple-Jack,” which is apple or cider brandy.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894