Definition: From around July 3 to August 11, summer really seems to take a very hot turn. Often the hottest part of the year in the Northern Hemisphere, this term is often used as slang for the high heat.
Named in part for the brightest star Sirius the Dog Star in the constellation Canis Major, the term dates back to the time of the ancient Egyptians. It was once believed the coincidence of the appearance of Sirius in the sky brought fertility to the Nile region. It was also believed by many primitive scholars to be part of the reason for the intense summer heat of this time frame.
The term should not be confused with sun dogs which are a physical phenomena which causes bright spots to appear on either side of the sun when it is near the horizon.
Reference National Weather Service Central Region Headquarters