とんでもない
とんでもない · tondemonai · JLPT N3 · i-adj
No way!
From 途でもない: not even on the road, so way off track
The usual etymology traces it to 途 (to, road or way) + でもない (not even), which became とんでもない. Something that has left the road of reason is outrageous, and said as a reply it means No way! or Not at all! Compare 途方もない, extraordinary.