litotes
noun [ U ] language specialized uk/laɪˈtəʊ.tiːz/ us/ˈlaɪ.tə.tiːz/
反叙法，曲言法（用否定对立的词语表示肯定的一种修辞格）
the use of a negative statement in order to emphasize a positive meaning, for example "a not inconsiderable amount of money (= a considerable amount of money)"