groin (n.) Look up groin at Dictionary.com
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.