The word "unreason" is a transparent English compound formed by directly applying the native Germanic negative prefix "un-" to the noun "reason." The core noun "reason" entered English from Old French raison, which itself evolved from Latin ratio, meaning a "calculation, account, or faculty of the mind." The Latin root is from the verb reri ("to think, calculate"), traceable to the Proto-Indo-European root \re- ("to reason, count"). Therefore, "unreason" literally means "not-reason" or "the absence of calculation/thought." Its formation follows a common pattern in English for creating opposites, and its meaning has remained closely tied to this literal sense, denoting irrationality or a lapse from logical thought.

Structure: un (not) + reason (logical thought) - un: A prefix of Germanic origin (Old English un-), meaning "not." It functions as a negative particle. - reason: A noun of Latin origin (via Old French raison), from Latin ration- (stem of ratio) meaning "reckoning, understanding, motive." It functions as the core substantive element.