Merriam-Webster's Collegiate® Dictionary, 11th Edition
GROIN
I. noun
Etymology: alteration of Middle English grynde, from Old English, abyss; akin to Old English grund ground
Date: circa 1532
the fold or depression marking the juncture of the lower abdomen and the inner part of the thigh,
2. the projecting curved line along which two intersecting vaults meet, a rib that covers this edge, a rigid structure built out from a shore to protect the shore from erosion, to trap sand, or to direct a current for scouring a channel,
II. transitive verb
Date: circa 1816
to build or equip with groins