Ironical understatement

• Figure of speech like "no mean feat"
• Understatement for effect
• Figure of speech that employs ironic understatement
• T S Eliot's quirky figure of speech
• Understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)
• Little child in stories making understatement
• Toilets revolting; and that's an understatement

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Meiosis \Mei*o"sis\ (m[-i]*[=o]"s[i^]s), n. [NL., fr. Gr. mei`wsis, fr. meioy^n to make smaller, from mei`wn. See Meionite .] (Rhet.) Diminution; a species of hyperbole, representing a thing as being less than it really is; understatement; see also litotes ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
rhetorical figure in which an affirmative is expressed by the negative of its opposite, from Greek litotes , literally "plainness, simplicity," from litos "smooth, plain, small, meager," from PIE root *(s)lei- "slimy, sticky, slippery" (hence "smooth"); ...