Saying the opposite of what you mean is a type of understatement called

Understanding Litotes

Litotes is a type of understatement that is used to emphasize a point by denying the opposite. It is an ironic form of expression where the speaker intends to highlight a particular idea by understating it. Instead of stating something directly, litotes uses negative phrasing to convey the intended meaning.

For example, if someone says, "It's not the best weather today," they are actually implying that the weather is quite bad. By denying that it is the best weather, they are indirectly emphasizing how poor the weather actually is, using litotes to make their point.

Litotes is often used for rhetorical effect, to add depth or subtlety to a statement, or to make a point more forcefully by understating it. It can create an air of sophistication or wit in communication when used effectively.

Overall, litotes is a powerful literary device that plays with understatement to convey a more impactful message. It adds layers of meaning to the words spoken or written, engaging the audience in a more nuanced way.