The Collaborative International Dictionary of English v.0.48:

Quoin \Quoin\ (kwoin or koin; 277), n. [See Coin, and cf.
   Coigne.]
   1. (Arch.) Originally, a solid exterior angle, as of a
      building; now, commonly, one of the selected pieces of
      material by which the corner is marked.
      [1913 Webster]

   Note: In stone, the quoins consist of blocks larger than
         those used in the rest of the building, and cut to
         dimension. In brickwork, quoins consist of groups or
         masses of brick laid together, and in a certain
         imitation of quoins of stone.
         [1913 Webster]

   2. A wedgelike piece of stone, wood, metal, or other
      material, used for various purposes; as:
      (a) (Masonry) To support and steady a stone.
      (b) (Gun.) To support the breech of a cannon.
      (c) (Print.) To wedge or lock up a form within a chase.
      (d) (Naut.) To prevent casks from rolling.
          [1913 Webster]

   Hollow quoin. See under Hollow.

   Quoin post (Canals), the post of a lock gate which abuts
      against the wall.
      [1913 Webster]

WordNet (r) 3.0 (2006):

quoin
    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]
    3: (architecture) solid exterior angle of a building; especially
       one formed by a cornerstone [syn: corner, quoin]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "quoin":
   Intertype, L, Linotype, Monotype, angle, apex, bend, bifurcation,
   bight, cant, chevron, coin, cold-type typesetting, composing,
   composing stick, composition, computerized typesetting, corner,
   cornerstone, crank, crook, crotchet, deflection, dogleg, dummy,
   first stone, footstone, fork, foundation stone, furcation,
   furniture, galley chase, headstone, hook, hot-metal typesetting,
   imposition, inflection, justification, keystone, knee, layout,
   line of type, nook, photocomposition, photosetting,
   phototypesetter, phototypesetting machine, point, setting, slug,
   swerve, typesetting, typesetting machine, veer, vertex, zag, zig,
   zigzag