[...] he had wedged himself in the coign of a double-stemmed meshwood trunk[...]
--On, by Adam Roberts, p. 225 of the 2002 Gollancz paperback edition
This one looks even more familiar than the last one; and even if I didn't know what it meant before, I could get a decent idea from context. So I almost didn't list it, but then I saw that MW3 lists it as a variant spelling for "quoin," which I didn't think I'd ever seen, and which means "angle" or "corner." It can also refer to a wedge, a keystone (or a "voussoir" (!)), or a lozenge (in the sense of a cut gem facet). Or the bricks (or other pieces) that make up the exterior corner where two walls meet; and actually, the illustration for that looks familiar, so maybe I've seen "quoin" before after all.
So that's interesting, but even more interesting is that MW3 lists two different entries for "coign"; the other says that it's derived from "coin," which turns out to be an archaic term for "corner" or "cornerstone" (from Middle French coin meaning "wedge," "stamp," or "corner"). And "quoin" derives from "coin."
Also showing up when I searched in MW3 for these words were phrases like "canting quoin" (also "canting coin"): "a triangular block for steadying stowed casks in a ship." And "coign of vantage": "a position advantageous for action or observation."
And in fact if you look up "coign" in MW11 (abridged), the only entry that comes up is "coign of vantage." So I guess the spelling "coign" is at least somewhat obscure after all.