E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Sinister (Latin, on the left hand).
According to augury, birds, etc., appearing on the left-hand side forbode ill-luck; but, on the right-hand side, good luck. Thus, corva sinistra (a crow on the left-hand) is a sign of ill-luck which belongs to English superstitions as much as to the ancient Roman or Etruscan. (Virgil: Eclogues, i. 18.)