adverb, adverb taking the 'to' particle
Describes the sound or action of eating something hard or crunchy, like crackers or raw vegetables.
彼はせんべいをボリボリ食べている。
He is munching on rice crackers.
Describes the sound or action of scratching skin, often vigorously.
蚊に刺されたところをボリボリかいた。
I scratched the mosquito bite vigorously.