First recalling an old proverb to justify his parsimony, Deasy says to Stephen, "If youth but knew." Then, reaching for the imprimatur of Great English Literature, he comes up with a phrase from the Bard himself: "But what does Shakespeare say? Put but money in thy purse." He does not notice Stephen's "murmured" reply, "Iago." The sentence that Deasy wants to use for validation comes from the mouth of Shakespeare's most famous villain, who aims to take all the money for himself. In Ithaca, we learn that Bloom too has sometimes succumbed to the folly of using Shakespeare's works as an owner's manual for human life.
Thornton cites the old proverb, "If youth but knew what age would crave, it would both get and save." "Put but money in thy purse" comes from Othello 1.3, in which Iago speaks the words repeatedly to Roderigo, urging him to sell all his lands and give the money to Iago, who will use it to convince Desdemona to have sex with Roderigo. This is hardly an instance of thrifty saving, and in any case Iago is a sociopath who is conning Roderigo out of all his wealth. (He remarks conspiratorially to the audience, "Thus do I ever make my fool my purse"). Deasy is once again far off the mark.
Deasy might have as easily quoted Polonius' famous words "Neither a borrower nor a lender be": his advice to Laertes has often been repeated uncritically. But then he would have identified himself with a foolish and hypocritical old man. Quoting from Shakespeare without reading him can be a tricky business.
Ithaca makes fun of citing Shakespeare as a moral authority, telling us that Bloom "reflected on the pleasures derived from literature of instruction rather than of amusement as he himself had applied to the works of William Shakespeare more than once for the solution of difficult problems in imaginary or real life." "Had he found their solution?" the catechism asks. "In spite of careful and repeated reading of certain classical passages, aided by a glossary, he had derived imperfect conviction from the text, the answers not bearing in all points."