Macbeth

From the Scottish play, description of the castle at Inverness. The birds love it. "Coign" = corner; viz., a useful corner. http://shakespeare.mit.edu/macbeth/macbeth.1.6.html

A reference to a scene in the Shakespeare play Macbeth, where Lady Macbeth has gone mad from guilt and obsessively washes her hands.

"Agenbite of inwit" is Middle English, translating to "prick of remorse". Stephen is referring sarcastically to those who bathe more often than he does --perhaps they are trying to whiten their unclean consciences? "Yet here's a spot" refers to Shakespeare's Macbeth, in which the crazed Lady Macbeth comes to believe that she cannot wash the blood of her murders off her hands.