The word whet derives from the German word, wetten, meaning to sharpen something, particularly a sword in preparation for battle. Later it began to be used figuratively – for example, to sharpen one's appetite or interest, which is how we use it today. Brutus uses whet in both its figurative and literal senses. Cassius has sharpened Brutus’ desire to overthrow Caesar, and Brutus sees himself as the sharpened weapon that will kill Caesar.