litotes

You probably use litotes every day. Ever say, "He's not the brightest bulb in the lamp," meaning "He's dumb"? Or "She's no beauty queen" meaning "She's ugly"? Well, those are examples of litotes — a way of saying something by saying what it's not.

Beware using litotes too often, especially in written form. George Orwell, who had a lot to say about the misuses of language, once suggested that a good cure for the excessive use of the "not un-" format (a classic litotes), as in "a not unintelligent person," was to memorize the following sentence: "A not unblack dog was chasing a not unsmall rabbit across a not ungreen field." It usually does the trick.

Definitions of litotes
1

n understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)

“saying `I was not a little upset' when you mean `I was very upset' is an example of litotes
Synonyms:
meiosis
Type of:
understatement
a statement that is restrained in ironic contrast to what might have been said
rhetorical device
a use of language that creates a literary effect (but often without regard for literal significance)