This is not a GQ. I can read a dictionary and I can Google.
So I know what the “real” definition is, but I seriously doubt that most people who use the term are speaking of the astronomic relevance.
If you use the term (and I don’t, so I can’t offer an opinion), what does it mean to you?
Here’s what Britannica says: “periods of exceptionally hot and humid weather that often occur in July, August, and early September in the northern temperate latitudes. The name originated with the ancient Greeks, Romans, and Egyptians; they believed that Sirius, the dog star, which rises simultaneously with the Sun during this time of the year, added its heat to the Sun’s and thereby caused the hot weather. Their belief that dogs were subject to spells of madness at this time also may have contributed to the name. Because people tended to become listless during the dog days, Sirius was held to have a detrimental effect on human activities.”
The underlines sentence was the explanation I was given growing up in the midwest.