Source : Webster's Revised Unabridged Dictionary (1913)

Coign \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.

         From some shielded nook or coign of vantage. --The
                                                  Century.

         The lithosphere would be depressed on four faces; . . .
         the four projecting coigns would stand up as
         continents.                              --Nature.

Source : WordNet®

coign
     n 1: expandable metal or wooden wedge used by printers to lock up
          a form within a chase [syn: {quoin}, {coigne}]
     2: the keystone of an arch [syn: {quoin}, {coigne}]