Gnof, used to introduce a major theme—our need to question even our most basic assumptions—in The Miller's Tale, is, remarkably, both a neologism and a borrowing. Because this essay sets out to prove that Chaucer created this word, the linguistic evidence is considered in detail. In Middle English, gnof appears only in the opening description of John the carpenter, it lacks a convincing etymology, and it was changed by several scribes as they copied their exemplars. In Early Modern English, it was revived by a limited number of authors, many of whom were associated with antiquarian interests in Chaucer's works, during the time of the inkhorn debate. Moreover, because I also argue that Chaucer took the sound of gnof but not its meaning from the Tuscan interjection gnaffé, which he found in Decameron, III.4, I consider the larger thematic links between these two narratives. If correct, this combined evidence leads to more than an interpretation of The Miller's Tale or new evidence for the reception of the Canterbury Tales. Chaucer did not just borrow lexemes from languages such as French and Latin and echo, on occasion, specific dialects of Middle English, but created new words. And, as an unusual verbal borrowing, gnof proves his use of the Decameron.