a favourable position for observation or action - literary
The literal sense of a coign of vantage is a projecting corner of a wall or building…the phrase appears in Shakespeare's Macbeth in Duncan's description of the nesting places of the swifts at Macbeth's castle. The word quoin meaning an external angle of a building still exists in English, but the archaic spelling coign survives mainly in this phrase.
Related Idioms :
I should cocoa = I should coco
I should say so – British rhyming slang
1996 - Melvin Burgess - Junk He said, 'Someone'll really buy it and it'll be theirs then.' 'I should coco,' I said.
bring something up to code
renovate an old building or update its features in line with the latest building regulations - North American