Litotes is a figure of speech consisting of an understatement in which an affirmative is expressed by negating its opposite. For example, instead of saying that someone is mean, you can say he is not very generous.
He's not a very generous man.
She is not very beautiful.
He is not the friendliest person I 've met.
Don't be too wicked.
It won't be an easy trip
He is not unaware of his wife's foolishness.