sinister

Sinister, sinistra, sinistrum is a first and second declension adjective of the -er type: the e drops in every form after the masculine nominative singular, so sinistri, not sinisteri. The feminine sinistra stands alone as a noun meaning the left hand, with manus understood, and the neuter plural sinistra means the left flank.

The second meaning is where the religion shows. Roman augury faced south, putting the east and its good omens on the left, so early Latin could treat sinister as favourable; Greek practice faced north and reversed it, and the Greek-influenced sense won, which is how left ended up meaning ill-omened. English sinister took only the bad half. Modern political left and right have nothing to do with any of this: they come from the seating in the French National Assembly of 1789.