What is dog water?

Dog water is often used in a derogatory way to describe something of very low quality. It is possible the term originates from actual dog's water bowls outside which, over time, can acquire algae, dirt, mold or other detritus from being outdoors and not changed or cleaned frequently. Alternatively, dog water sometimes has been used to compare water to toilet water or something worse and less than human-drinkable and thus, inferior and low quality.