The word 'unreasonable' is a layered construction built upon the core concept of 'reason'. Its journey begins with the Latin ratiōnem, meaning "calculation, account, understanding," which entered English via Old French as 'reason'. The suffix '-able', from Latin -ābilis, was attached to form 'reasonable', meaning "endowed with reason" or "fair, moderate." Finally, the Old English negative prefix 'un-' was prefixed to negate the entire concept, creating a term that literally means "not capable of being reasoned with" or "not possessing sound judgment." This logical assembly from Latin and Germanic elements precisely captures the modern sense of being illogical, unfair, or excessive.

Structure: un (not) + reason (cause, logic) + able (capable of) - un: A prefix of Germanic origin (Old English un-), meaning "not." It functions as a negative particle. - reason: A root of Latin origin (ratiōnem, "reckoning, understanding"), functioning as the core noun meaning "logic, cause, justification." - able: A suffix of Latin/French origin (-ābilis), meaning "capable of, worthy of." It forms adjectives meaning "having the quality of."