Litotes
Litotes is a language device where you combine a negative word (like "no", "not", "don't", "isn't", etc) with a word or phrase to express the opposite thing for effect. For example, "I'm not happy" means you're unhappy.
Another interesting example of litotes comes from A Streetcar Named Desire when Blanche describes the Flamingo as 'not the sort of establishment [she] would dare to be seen in'. It's an understatement of the fact that the Flamingo is a really disreputable hotel!