Roman augurs, when studying the heavens for signs which, it was believed, would indicate the will of the gods, faced toward the south.
The eastern heavens from which favorable omens were expected, were therefore upon the left; unfavorable omens in the west or toward the right.
Thus among the Romans, sinister, which means left or left-hand, meant lucky, favorable, or auspicious, and dexter, which means right or right-hand, meant unlucky or inauspicious. But Greek augurs faced the north when looking for signs from the heavens.
The eastern or favorable omens, accordingly, appeared on the right side, and the unfavorable ones upon the left. Roman poets, who almost invariably aped Greek customs, followed the Greek thought in this field as well.
Hence, because the writings of the Roman poets were popular in the literary world of England in the sixteenth and seventeenth centuries, sinister, although a Latin word, has come down to us in the sense ascribed by the Greek augurs to the left or western side of the heavens; hence, of evil aspect.