litotes
noun
li·to·tes ˈlī-tə-ˌtēz 
ˈli-,
lī-ˈtō-ˌtēz plural litotes
: understatement in which an affirmative is expressed by the negative of the contrary (as in "not a bad singer" or "not unhappy")
Did you know?
Litotes Probably Pop Up in Your Everyday Speech
Even if you've never heard the word litotes, chances are you've encountered this figure of speech. If you've ever approved of a job well done by exclaiming "Not bad!" or told someone that you are "not unhappy" when you are ecstatic, you've even used it yourself. In fact, you might say that it would be "no mean feat" to avoid this common feature of our language! And litotes isn't only common; it's also simple—etymologically speaking, that is. Litotes evolved from a Greek word meaning "simple," and perhaps ultimately from another Greek word meaning "linen cloth."
Word History
Etymology
Greek litotēs, from litos simple, perhaps from lit-, lis linen cloth
First Known Use
1589, in the meaning defined above