noun

1. the external corner of a building; esp., any of the large, squared stones by which the corner of a building is marked
2. a wedgelike piece of stone, etc., such as the keystone or one of the pieces of an arch
3. a wedge-shaped wooden or metal block used to lock up type in a galley or form, to keep casks from rolling, etc.

Etymology: var. of coin

transitive verb

1. to secure with a quoin
2. to furnish with quoins, or corners