DIMINUTIO

This is a rhetorical device used by speakers and by Chaucer in The Canterbury Tales whereby a character disparages his own skills, often claiming to be a blunt person, incapable of elevated discourse. This allows him to be crude or, alternatively, impressive with poetic language which comes as a surprise. The Franklin says he is a "burel" [plain, homely] man and yet tries to win admiration with a Tale about aristocrats with some rhetorical flourishes.  The device may still be heard when someone starts a speech: "Unaccustomed as I am to public speaking."