groin (n.)

"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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Definitions of groin from WordNet
groin (n.)
the crease at the junction of the inner part of the thigh with the trunk together with the adjacent region and often including the external genitals;
Synonyms: inguen
groin (n.)
a curved edge formed by two intersecting vaults;
groin (n.)
a protective structure of stone or concrete; extends from shore into the water to prevent a beach from washing away;
Synonyms: breakwater / groyne / mole / bulwark / seawall / jetty
groin (v.)
build with groins;
The ceiling was groined