"A plea in an action of assumpsit by which the defendant denies having made any promise or undertaking", Mid 17th cent.; earliest use found in Reports of Cases of the Star Chamber and High Commission. From post-classical Latin non assumpsit he did not undertake from classical Latin nōn not + assūmpsit he undertook, 3rd singular perfect indicative of assūmere.[non assumpsit etymology, non assumpsit origin, 英语词源]