Meaning

noun a partial excuse to mitigate censure; an attempt to represent an offense as less serious than it appears by showing mitigating circumstances

noun to act in such a way as to cause an offense to seem less serious

originAn adaptation of extenuātiōn-, the oblique stem of the Latin extenuātiō (“a thinning or diminishing”, “rarefaction”; rhetoric “a lessening”, “diminution”, “extenuation”), noun of action from extenuō (“to thin, reduce, or diminish”). Equivalent to extenuate + -ion. Compare the French exténuation.

Etymology from Wiktionary, CC BY-SA 3.0