dog star (n.)

brightest star by magnitude, late 14c., from Latin Sirius "the Dog Star," from Greek Seirios, said to mean literally "scorching" or "the scorcher." But other related Greek words seem to derive from this use, and the name might be a folk-etymologized borrowing from some other language. An Egyptian name for it was Sothis. Beekes suggests it is from PIE root *twei- "to agitate, shake, toss; excite; sparkle" if the original meaning of the star-name is "sparkling, flickering."

The connection of the star with scorching heat is due to its ancient heliacal rising at the summer solstice (see dog days). Related: Sirian (1590s). The constellation Canis Major seems to have grown from the star.

Homer made much of it as [Kyōn], but his Dog doubtless was limited to the star Sirius, as among the ancients generally till, at some unknown date, the constellation was formed as we have it, — indeed till long afterwards, for we find many allusions to the Dog in which we are uncertain whether the constellation or its lucida is referred to. [Richard Hinckley Allen, Canis Major in "Star Names and Their Meanings," London: 1899]

"period of dry, hot weather at the height of summer," 1530s, from Latin dies caniculares, the idea, though not the phrase, from Greek; so called because they occur around the time of the heliacal rising of Sirius, the Dog Star (kyōn seirios). Noted as the hottest and most unwholesome time of the year; often reckoned as July 3 to August 11, but variously calculated, depending on latitude and on whether the greater Dog-star (Sirius) or the lesser one (Procyon) is reckoned.

The heliacal rising of Sirius has shifted down the calendar with the precession of the equinoxes; in ancient Egypt c. 3000 B.C.E. it coincided with the summer solstice, which also was the new year and the beginning of the inundation of the Nile. The "dog" association apparently began here (the star's hieroglyph was a dog), but the reasons for it are now obscure.