'unreasonableness' originates from the combination of 'un-' meaning 'not' and 'reasonable,' which comes from Latin 'rationabilis,' meaning 'reasonable or logical.'

'unreasonableness' evolved from the Middle English word 'unresonable,' which was derived from Old French 'unraisonable,' and eventually became the modern English word 'unreasonable.'

Initially, it meant 'not governed by reason,' and this meaning has largely remained the same in modern usage.