The story
- Latin · derived fromirratiōnābilis
- Middle English · inherited fromunresonable
- English todayunreasonable
From Middle English unresonable, equivalent to un- + reasonable, originally partly after Latin irratiōnābilis.
Words from the same roots
Latinirratiōnābilis all →
irrationable
Etymology from Wiktionary, CC BY-SA 4.0.