Sinistra isn't the Latin for left, it's the feminine nominative singular of sinister, which means left. In Roman augury the augur faced south, so the fortunate eastern side of the sky was on his left. In Greek augury the augur faced north, which meant that the lucky side was on his right. Augustan poets and their immediate predecessors (most notably Catullus) began to use the adjective sinister to mean unfavorable, since they were using Greek verse forms. The idiom spread from verse to prose, where the word had been used to mean favorable in Cicero's time, in the Silver Latin period.