extenuate
Concise Oxford English Dictionary © 2008 Oxford University Press:
extenuate /ɪkˈstɛnjʊeɪt, ɛk-/
▶verb (usu. as adj. extenuating) lessen the seriousness of (guilt or an offence) by reference to a mitigating factor.
– derivatives
extenuation noun,
extenuatory /-jʊət(ə)ri/ adjective.
extenuation noun,
extenuatory /-jʊət(ə)ri/ adjective.
– origin ME (in the sense ‘make thin’): from L. extenuat-, extenuare ‘make thin’, based on tenuis ‘thin’.
'extenuate' also found in these Oxford entries:

