Today it means: A telescopic binary star, visually the brightest star in the night sky, a part of the northern constellation of Canis Major (the Greater Dog), one of three stars in the Winter Triangle asterism. Long understood as a single extremely luminous white star, it was associated in ancient Egypt with the Nile flood and in Greek and Roman culture with the "dog days" of summer. Full definition →
From Latin Sīrius, from Ancient Greek Σείριος (Seírios), usually taken from σείριος (seírios, “scorching; scorcher”).
Etymology from Wiktionary, CC BY-SA 4.0.