What is the Meaning of the Idiom WORK LIKE A DOG?

to work very hard
Examples:

1. He has two jobs and he goes to university. He is working like a dog to give his children a better future.

2. I wish my sister had a better job. The pay at her job is terrible, and her boss works her like a dog.

3. We are going to have to work like dogs to get our house clean this weekend. Relatives are coming to visit on Monday, and we have been painting. Our place is a mess!