The word "unreasoning" is built upon the core concept of "reason," which traces back to the Latin ratio, meaning a reckoning, calculation, or logical faculty. The suffix "-ing" is added to form an adjective describing something characterized by that action—in this case, "reasoning." Finally, the Old English negative prefix "un-" is attached to negate the entire concept. Thus, the literal construction "not reasoning" directly and logically yields the modern abstract meaning of acting without or devoid of logical thought, capturing a state of irrationality or instinctive reaction.

Structure: un (not) + reason (think logically) + ing (adjective suffix) - un: A prefix of Germanic origin (Old English) meaning "not," serving a negating function. - reason: A root of Latin origin (ratiōnem, "calculation, reasoning"), serving as the core noun denoting logical thought. - ing: A suffix of Germanic origin (Old English -ende, -ing) forming present participles and adjectives from verbs, here indicating a quality or state.