litotes

Etymology

From Late Latin lītotēs, from Ancient Greek λιτότης (litótēs, literally “plainness”), from λιτός (litós, “simple”).

noun

1. (rhetoric) An ironic figure of speech whereby something is stated by denying its opposite, particularly the negation of a negative quality to say something positive.
   The delicacy which prompts a later generation to reject that name is by no means necessarily a result of stricter habits, is far more often due to the flatness which comes of untiring repetition and to the greater piquancy of litotes. 1895, William Congreve, “Introduction”, in G. S. Street, editor, The Comedies of William Congreve, volume 1, Methuen and Co.