Litotes are rhetorical figures of speech used in poetry and prose in which emphasis is gained by understated negation. The phrase, "He's not a rocket scientist," could be considered an example.
The word litotes comes from the Greek "litos," meaning "plain or meager." The generally accepted idea behind litotes as a literary device is that they can allow the writer or speaker to make an unpleasant or negative statement in a discreet and indirect manner. Litotes have historically been a part of the Old English and Germanic language family poetry traditions. The Icelandic sagas, for example, provide many examples of litotes.