"I WORKED LIKE A DOG" - WHAT?

Sammie is always complaining to Hun that she worked like a dog in the garden or she worked like a dog cleaning the house.  He just gives her his usual blank stare.  I’ve always wondered though about that expression, worked like a dog, because I don’t know any dogs that work.  I’ve never seen any of my canine friends doing hard labor around the neighborhood.  My job description says I’m supposed to look cute and wag my tail once in a while.  Okay, I wrote it, but it works for me.