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Phrase(s): work like a beaver [and] work like a mule; work like a horse; work like a slave
Fig. to work very hard. • She has an important deadline coming up, so she’s been working like a beaver. • You need a vacation. You work like a slave in that kitchen. • I’m too old to work like a horse. I’d prefer to relax more.
McGrawhill's American Idioms And Phrasal Verbs
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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.
American Heritage Idioms