coign

coign [kɔɪn] n.

1.) In the Shaksperian phrase 'coign of vantage': a position (properly a projecting corner) affording facility for observation or action. (The currency of the phrase is apparently due to Sir Walter Scott.)
2.) Occasionally used in the following senses, where 'quoin' is the ordinary modern spelling: a corner-stone; a projecting corner or angle of a building; a wedge (in Printing or Gunnery).
3.) Geology. An original angular elevation of land around which continental growth has taken place (O.E.D. 2nd Ed.).

Etymology: An archaic spelling of 'coin', 'quoin', retained chiefly in connexion with the phrase in (1). 'Coin' is an adoption of the French coin, wedge, corner; also die for stamping money or medals (so called because the die had the form or action of a wedge).

"This guest of summer,
The temple-haunting martlet, does approve,
By his loved mansionry, that the heaven's breath
Smells wooingly here: no jutty, frieze,
Buttress, nor coign of vantage, but this bird
Hath made his pendent bed and procreant cradle:
Where they most breed and haunt, I have observed,
The air is delicate."
(The Tragedy of Macbeth, William Shakespeare, 1623).

(A Coign of Vantage, Lawrence Alma-Tadema, 1895)