The word 'unreasonableness' is built upon the foundational Latin concept of ratio, which originally meant a "reckoning" or "calculation" and evolved to signify the faculty of logical thought. The suffix -able was attached to form 'reasonable,' meaning "endowed with reason" or "conforming to logic." The Germanic negative prefix un- then reverses this quality, creating 'unreasonable' to describe actions or judgments lacking logical foundation. Finally, the abstract noun-forming suffix -ness, also of Germanic stock, is appended to crystallize this negative quality into a state or attribute, resulting in the modern term denoting a lack of fairness or sound judgment.

Structure: un (not) + reason (cause, justification) + ableness (quality or state of being) - un: A prefix of Germanic origin (Old English un-), meaning "not." It functions as a negative particle. - reason: A root of Latin origin (ratiōnem, accusative of ratiō, meaning "reckoning, understanding, motive"). It functions as the core noun denoting logic or justification. - ableness: A suffix complex of Germanic origin, combining -able (from Latin -ābilis, meaning "capable of, fit for") and -ness (from Old English -nes, forming abstract nouns). It functions to create a noun indicating the quality or state of being capable of the root's action.