Coign

n. A var. spelling of Coin, Quoin, a corner, wedge; -- chiefly used in the phrase coign of vantage, a position advantageous for action or observation. [ Webster 1913 Suppl. ]

From some shielded nook or coign of vantage. The Century. [ Webster 1913 Suppl. ]

The lithosphere would be depressed on four faces; . . . the four projecting coigns would stand up as continents. Nature. [ Webster 1913 Suppl. ]

2. an expandable metal or wooden wedge used by printers to lock up a form within a chase.
Syn. -- quoin, coigne. [ WordNet 1.5 ]

3. the keystone of an arch.
Syn. -- quoin, coigne. [ WordNet 1.5 ]