both groin and quoin are wedges..

but they surely have completely different etymologies?

groin (groin)
n.
1. Anatomy. 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.
2. Architecture. The curved edge at the junction of two intersecting vaults.
3. A small jetty extending from a shore to protect a beach against erosion or to trap shifting sands.
tr.v., groined, groin·ing, groins.
To provide or build with groins.

Alteration (influenced by LOIN) of Middle English grinde, perhaps from Old English grynde, abyss, hollow.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2000 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.



edit: hmm, no, I see the areas of overlap, but must admit I still don't follow the entire picture - need to coin some more time to study it!