1. one of the stones forming an external wall; cornerstone.
2. an external solid angle of a wall or the like.
3. any of various bricks of standard shape for forming corners of brick walls or the like.
4. a wedge-shaped piece of wood, stone, or other material, used for any of various purposes.
5. Printing. a wedge of wood or metal for securing type in a chase.
verb: 1. to provide with quoins, as a corner of a wall. 2. to secure or raise with a quoin or wedge.
He learned how to fashion an ornate quoin corner in many styles, before he settled on the unadorned quoin corner that is his trademark. — Walt Harrington, At the Heart of It, 1996
See you yon quoin o’th’ Capitol, yon cornerstone? — William Shakespeare, The Tragedy of Coriolanus, 1608
Quoin, which entered English in the sixteenth century, shares its root with the word coin. They both come from the Latin cuneus meaning “a wedge.”