Maura: To work like a dog means to work very hard.
Harp: Yes. So if you use the expression to work like a dog, that means that someone is working very hard, putting in a lot of effort.
Maura: Whenever we have an idiom with an animal, I always wonder, “Why that animal?”
Harp: And Maura, why a dog?
Maura: Well, dogs do work hard. And I’m not talking about the dogs that most people have for pets, but dogs that work, like a seeing-eye dog that helps people who are visually impaired or the dogs that pull sleds, they work pretty hard too.
Harp: Yeah, and police dogs work really hard too.
Maura: Right. So a dog that works, works almost all the time.