A rhetorical term for understatement is litotes:
litotes [LY-tuh-teez] (noun): understatement in which an affirmative is expressed by the negative of the contrary (as in “He’s not a bad ballplayer”)—Merriam-Webster.
Litotes can be used to express a variety of meanings.
When the translators of the KJV have Paul of Tarsus identify himself to the Roman officer as “a citizen of no mean city” (Acts 21:39), it is not to suggest that Paul was being modest. The words are “no mean city,” but the meaning is “a very important city.”
Here are other examples of the use of litotes to emphasize the importance of something by using a negative to express the contrary:
The history of American freedom is, in no small measure, the history of procedure. (i.e., “to a great extent”)
The disparity in government funding is not easy to remedy. (i.e, “extremely difficult”)
Litotes is also used to convey modesty, sarcasm, contempt, admiration, and veiled disapproval, as in the following examples:
You’ve managed to wreck the car and destroy the front porch all in one go. Good job!
He’s no Einstein.
Oprah gave every guest a car? Not too shabby.
This day-old lobster bisque is not entirely inedible.
Understatement has been a popular form of expression in English since the earliest times. For example, the Old English epic Beowulf begins with a gory description of Grendel’s slaughter of thirty of Hrothgar’s thanes. Grendel seizes the thanes and carries some of the bloody bodies back to his lair, “exulting.” Later, Grendel returns to wreak more slaughter. Says the poet, “[The monster] did not mourn for it.”