Coign of vantage
In Proteus Stephen imagines his aunt's family spying out at a supposed "dun" (a bill collector) "from a coign of vantage," i.e. an advantageous corner. The phrase comes from Shakespeare's Macbeth, in the scene in which King Duncan and his cohort ride up to Macbeth's castle, and Stephen's mulling of the phrase seems to have some influence on his decision not to pay the Gouldings a visit.
The King remarks that "This castle hath a pleasant seat" and that the air strikes the senses "Nimbly and gently." Banquo agrees that "the heaven's breath / Smells wooingly here," and he observes that martins nest all over the walls of the castle: "no jutty, frieze, / Buttress, nor coign of vantage, but this bird / Hath made his pendant bed and procreant cradle. / Where they most breed and haunt, I have observ'd / The air is delicate" (1.6.1-8).
The thought of birds peering out of their pendant (i.e., hanging) beds pops into Stephen's mind, perhaps quite innocuously, when he thinks of his aunt's family peering down from cracks in the shuttered windows to see if a "dun" is at the door. But it is hard to ignore the context in which Banquo utters his phrase. King Duncan is entering the castle where he will soon be slaughtered, in his bed, by his hosts. Moments earlier, Lady Macbeth has invoked a less benign bird to herald his arrival: "The raven himself is hoarse / That croaks the fatal entrance of Duncan / Under my battlements" (1.5.38-40). And immediately after Duncan and Banquo speak, she enters to welcome him under her battlements.
Stephen may not fear being butchered in his aunt's house, but the allusion does suggest a certain hesitation to enter the shuttered domicile. He has just been savoring the breezes on the beach: "Airs romped round him, nipping and eager airs." After thinking for a good long while of the kind of scene that will likely greet him inside the house, he decides not to sample its air: "This wind is sweeter." Heaven's breath does not smell so wooingly in Strasburg Terrace. (After thinking that the wind smells sweeter on the beach, Stephen goes on to think "Houses of decay, mine, his, and all. You told the Clongowes gentry you had an uncle a judge and an uncle a general in the army. Come out of them, Stephen. Beauty is not there.")
Later, in Scylla and Charybdis, Stephen again muses on the passage from Macbeth. Looking on Dublin's houses, he thinks: "Kind air defined the coigns of houses in Kildare street. No birds."