litotes (n.)

rhetorical figure in which an affirmative is expressed by the negative of its contrary ("no laughing matter"), from Greek litotes "plainness, simplicity," from litos "smooth, plain," also "frugal, small, meager," and, of style, "simple, unadorned," from PIE root *(s)lei- "slimy, sticky, slippery" (hence "smooth"); see slime (n.).