T S Eliot's quirky figure of speech

• Figure of speech like "not unlike"
• 'Not unhappy,' e.g
• using ___: “Mary had no giant lamb / Its fleece was not unwhite”
• Little child in stories making understatement
• Figure of speech that employs ironic understatement
• Understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary)
• Understatement for effect

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"); ...

Wikipedia Word definitions in Wikipedia
In rhetoric , litotes (, or ) is a figure of speech wherein understatement is used to emphasize a point by stating a negative to further affirm a positive, often incorporating double negatives for effect. For example, "She's not bad looking" could be used ...