12 letters · 17 points in Scrabble
adverb not in a reasonable or intelligent manner
adverb to a degree that exceeds the bounds or reason or moderation
originFrom Middle English unresonably, equivalent to unreasonable + -ly, after resonably (whence reasonably) and (originally) Late Latin irratiōnābiliter.
Etymology from Wiktionary, CC BY-SA 3.0