What does the phrase “dog days of summer” mean and where did it originate?

The dog in the phrase comes from the ancient Egyptians who named the brightest star in the night sky, Sirius, after the god Osiris whose head was similar to a dog's. The Egyptians believed that the dog star Sirius added heat to the sun and was responsible for extreme hot weather. They also thought that the extreme heat caused the Nile River to flood. And so it was passed down generations to mean the very hot days in summer.