From Longman Dictionary of Contemporary Englishbe hard at it/workbe hard at it/workinformal to be very busy doing something Sarah was hard at it on her computer. → hard
Examples from the Corpus
be hard at it/work• When she was hard at work and on top of things her productivity was exceptional.• Not much is said, as each young person, and Bill, is hard at work at the task at hand.• Thacker had set him a spot of overtime and he was hard at it in the mill.• Today, all eight of the Van Andel and DeVos offspring are hard at work making this company better.• He was hard at work on the translation of a play which had to be ready two days later.• Over the road, Sylvia Brackley and daughter, Karen are hard at work on this year's crop.• Ahead of her, Bite the Bullet's jockey was hard at work while the horse on his outside was clearly beaten.• Cook was making fresh cornbread rolls for breakfast and lesser mortals were hard at it with brooms and mops. 