adverb, adverb taking the 'to' particle
vigorously; briskly; boisterously
Describes doing something with energetic, rough, or forceful movements, often with a sense of noisiness or lack of restraint. Common in casual contexts.
彼はガシガシと頭をかいた。
He scratched his head vigorously.
子供たちがガシガシ遊んでいる。
The children are playing boisterously.