The term "mill dog" refers to a breed of dog that is specifically bred and raised for use in an animal mill, such as a puppy mill or a kennel. These animals are often kept in cramped and unsanitary conditions with little to no attention paid to their physical or emotional well-being. They may be bred repeatedly without regard for their health or the genetic quality of their offspring, resulting in a wide range of health issues that can affect both them and their puppies. The term "mill dog" is often used as a negative connotation to describe these animals and the conditions they are kept in.