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). Altered 16c. by influence of loin or obsolete groin "snout." The architectural groin "edge formed by the intersection of two vaults" is from 1725.
The crease or hollow at the junction of the inner part of each thigh with the trunk, together with the adjacent region and often including the external genitals.