Helen often referred to her husband as "milquetoast" because of his timid, unassertive nature and the fact that he was easily dominated by her more robust personality. Why is the word milquetoast in quotation marks?

Quotation marks are for quoting the exact words that someone else use. Quotation marks around a single word is used for laying emphasis on a word when quoting the exact word that someone else used. In the question above the word, 'Milquetoast' is put in quotation marks by the writer because it was the exact words that Helen used when she was referring to her husband.