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 from its ancient heliacal rising at the summer solstice (see dog days). Related: Sirian. The constellation Canis Major seems to have grown from the star, not the other way.
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]