[ks2048]

In Wiktionary,

https://en.wiktionary.org/wiki/sinister#Latin

it shows a few definitions of "sinister" in Latin that seem contradictory. "left", "perverse/bad", but "auspicious" for Romans, while "inauspicious" for Greeks. And a Proto-Indo-European source which is positive.

[Telemakhos]

The PIE source is positive, because it was applied as a euphemism in Latin to what would have been laevus (cf. Greek λαιός), from the PIE word for "left." The Greeks, too, preferred euphemism to the direct term for "left": the much more common term ἀριστερά ("left") is a constrastive/comparative derived from ἄριστος, "best," so it means the "bester side."

[card_zero]

Noa-names, a precaution against things like accidentally summoning a bear: https://en.wikipedia.org/wiki/Noa-name

[ks2048]

Interesting. I knew Spanish izquierda (left) came from Basque, but I didn't realize it was to avoid a taboo word like sinister.

[card_zero]

These are euphemisms to avoid another euphemism, as the other poster says, sinister itself starting out as positive. The ancient euphemism treadmill.