What does "Dog Days of Summer" mean?

We're in the "Dog Days of Summer" and according to our dogs, it's the time of the summer they don't want to go outside, and instead stay in the air conditioning.
As for the origin of the phrase, it goes back to the ancient Greeks and Romans who saw Sirius, the dog star in the summer time, and the star rose in the sky around the same time as the sun. (That doesn't happen any more as a result of change in the Earth's orbit and wobble). It was also thought to bring about evil in the world.
Nowadays, it means slow, hot, lazy days that run from July to September! Stay Cool!