Where does the word "sinister" come from?

Sinister did not start out evil. It started out as a direction.

In Latin, sinister meant "on the left side," and its opposite was dexter, "on the right." Etymonline says the origin of the Latin word itself is uncertain. A proposed link to a Sanskrit word meaning "more useful" is quoted there with a scholar's verdict that it is "gratuitous," because the meaning is not specific enough.

The bad reputation came from reading omens. According to Etymonline, Greek practice treated birds seen on the left as unlucky, and that influence gave the Latin word the sense of "harmful" or "adverse." Genuine Roman augurs faced south, which put the favorable side on their left. So Latin also kept a meaning of "lucky," and the word was never simply negative at home.

English took the word from Old French, where senestre or sinistre meant "contrary, false, unfavorable" as well as "to the left." It appears in English in the early 15th century as sinistre. The first uses were about things prompted by malice or ill will, such as a false report or a misleading suggestion. Merriam-Webster dates the sense "singularly evil or productive of evil" to the same century.

The sense "deceitful" applied to people shows up in the late 15th century. The plain meaning "left side of the body" is recorded in English from about 1500, which means the literal sense reached English after the bad ones.

That literal sense survives in heraldry. From the 1560s, sinister has meant "to the left" as seen from the person carrying the shield. A bend sinister, a diagonal stripe running the opposite way to the usual one, came to signal illegitimate birth. Etymonline stresses that the term is "bend sinister," not "bar sinister."

The partner word went in the other direction. Dexter gave English dexterity, skill with the hands. Etymonline notes that dexter carries a comparative ending, so it originally suggested "the better direction."

Even the plain English word left may carry a slight insult. Etymonline says it may come from an Old English word meaning "weak" or "foolish."

So the Latin word was a direction and an omen, and English kept only the bad half of the omen.