A sudden power of heat from the sun emerging from a cloud; Eastern England.--James Halliwell's Dictionary of Archaic and Provincial Words, 1855
Dog Days of Summer
These begin on July 3 and continue to August 11. They derive their name from the heliacal rising and setting of Sirius, the dog-star. ... We must look to Egypt for the origin of the observance of these days. The rising of Sirius coincided in ancient times with the summer solstice and the overflowing of the Nile, and as [this flooding] was considered the source of the fertility of Egypt, the period was regarded as sacred, and the influence of the dog-star was deemed peculiarly auspicious. The superstititous feelings generated in Egypt with regard to the dog-days gradually spread throughout the world, and made themselves felt. But, while the rising of the dog-star was the harbinger of prosperity to the Egyptian, it was just the reverse to the Roman, who looked upon the dog-days as unfortunate ... coming as they did in the most unhealthy period of the year.--William Walsh's Curiosities of Popular Customs, 1897
Well, when it comes to the ancients, I'm generally pretty pro-Egypt, but I gotta go with the Romans on this one.