Litotes (li-to’-tees): Deliberate understatement, especially when expressing a thought by denying its opposite. The Ad Herennium author suggests litotes as a means of expressing modesty (downplaying one’s accomplishments) in order to gain the audience’s favor (establishing ethos).
I’m not he funniest person in the world, and I haven’t earned millions in Las Vegas. But, I can make you laugh so hard you’ll wet your pants. Are you ready?
Definition courtesy of “Silva Rhetoricae” (rhetoric.byu.edu)