adverb
ridiculously; unusually; exceptionally
Used adverbially to emphasize an extreme degree, often with a nuance of surprise or mild criticism. Can be used both positively and negatively.
今日は馬鹿に暑いね。
It's ridiculously hot today, isn't it?
彼は馬鹿に早く走る。
He runs unusually fast.
馬鹿 is a noun/adjective meaning 'fool' or 'stupid', while 馬鹿に is its adverbial form meaning 'ridiculously' or 'unusually'.
Adverbial form of 馬鹿 (fool, stupid). The origin of 馬鹿 is uncertain, but it is a common word in modern Japanese.