greengage(n = noun.food) greengage plum - sweet green or greenish-yellow variety of plum;
is a kind of plum

   A kind of plum of medium size, roundish shape, greenish flesh, and delicious flavor. It is called in France Reine Claude, after the queen of Francis I. See Gage. [1913 Webster]

greengage, n. a roundish green fine-flavoured variety of plum.

Etymology:
Sir W. Gage d. 1727

greengage plum