Understatement was a way of describing something that made it seem less important, serious, or bad than it actually was.
In 2267, Leonard McCoy described the society on Argelius II as completely hedonistic. When Captain James T. Kirk commented on how they "think very highly of their pleasure," Doctor Leonard McCoy rebuffed, "that's an understatement if I ever heard one." (TOS: "Wolf in the Fold")