"oblique depression of the body between the abdomen and thighs," 1590s, earlier grine (1530s), from Middle English grynde "groin" (c. 1400), originally "depression in the ground," from Old English grynde "abyss," perhaps also "depression, hollow," from Proto-Germanic *grundus (see ground (n.)). Altered 16c. by influence of loin or obsolete groin "snout of a pig." The architectural groin "curving edge formed by the intersection of two vaults" is from 1725.
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