Sinister as Lucky

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