The Collaborative International Dictionary of English v.0.48:

Coign \Coign\ (koin), 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]

WordNet (r) 3.0 (2006):

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