work like a beaver
Also, work like a dog or horse or Trojan. Work very energetically and hard, as in She worked like a beaver to clean out all the closets, or I've been working like a dog weeding the garden, or He's very strong and works like a horse. The first of these similes is the oldest, first recorded in 1741; the variants date from the second half of the 1800s. Also see work one's fingers to the bone.