An adverb meaning 'ridiculously' or 'extremely' (used colloquially to indicate an excessive degree).
この靴、馬鹿に高かったから買うのをやめた。
These shoes were ridiculously expensive, so I decided not to buy them.
今日は馬鹿に暑いね。クーラーをつけよう。
It's absurdly hot today. Let's turn on the air conditioner.
彼の新車は馬鹿に速くて、運転が怖いくらいだ。
His new car is ridiculously fast — it's almost scary to drive.
そんなに馬鹿に心配しなくていいよ。
You don't need to worry so ridiculously much.