In ancient times, augurs (augures ex caelo) faced south, so the happy orient, where the sun rose, lay at their left. Consequently, the word sinister (Latin for left) meant well-fated. When, under Greek influence, it became customary for augurs to face north, sinister came to indicate the ill-fated west, where light turned into darkness. It is this latter and later meaning that is attached to the English word sinister.

Source: Glossary of ancient Roman religion – Wikipedia