(n): something solid that is usable as an inclined plane (shaped like a V) that can be pushed between two things to separate them
(n): the keystone of an arch
(n): the central building block at the top of an arch or vault
(n): (architecture) solid exterior angle of a building; especially one formed by a cornerstone
(n): a thing constructed; a complex entity constructed of many parts
Haha, now that the WordNet definition is hovering next to the word my comment looks redundant.
Interesting. It is also a letterpress term for a small, wedge-shaped piece of metal with teeth on one side. Used in sets of two, with their teeth facing eachother, quoins form an expandable rectangle that sits between blocks of wood (called furniture) on the press bed, and are tightened with a quoin key to secure type in place for printing.
I think this word also describes an angular block of wood, with a "handle" on one end, placed under the butt of a cannon barrel to to help aim it downward. When the quoin is removed, the barrel rests on the gun carriage and points skyward--not directly, but at too high an angle to fire effectively.