medlar (n.)

small fruit-bearing tree related to the crab-apple, c. 1400 (mid-14c. in reference to the fruit itself, earlier medle, c. 1300), from Old French medler, meslier, variants of mesple, from Latin mespila "fruit of the medlar," from Greek mespilion, a foreign word of unknown origin (Beekes thinks it probably Pre-Greek on account of the suffix).

"When first gathered, it is harsh and uneatable, but in the early stages of decay it acquires an acid flavor much relished by some" [Century Dictionary]. The tree was introduced into southern Europe from western Asia. In Romanic the initial consonant has shifted to n-; as in French nèfle, Spanish nespera, Italian nespolo (see napkin). The Old English name for the fruit was openærs, literally "open-arse," probably so called for the large puckered "eye" between the calyx lobes.