sinister (adj.)

early 15c., sinistre, "prompted by malice or ill-will; false, dishonest, intending to mislead," with suggestion, report, etc., from Old French senestre, sinistre "contrary, false; unfavorable; to the left" (14c.), from Latin sinister "left, on the left side" (opposite of dexter), a word of uncertain origin. Perhaps meaning properly "the slower or weaker hand" [Tucker], but Klein and Buck suggest it's a euphemism (see left (adj.)) and connected with the root of Sanskrit saniyan "more useful, more advantageous." With contrastive or comparative suffix -ter, as in dexter (see dexterity).

The Latin word was used in augury in the sense of "unlucky, unfavorable" (omens, especially bird flights, seen on the left hand were regarded as portending misfortune), and thus sinister acquired a sense of "harmful, unfavorable, adverse." This was from Greek influence, reflecting the early Greek practice of facing north when observing omens. In genuine Roman auspices, the augurs faced south and left was favorable. Thus sinister also retained a secondary sense in Latin of "favorable, auspicious, fortunate, lucky."

In reference to persons, "deceitful, perfidious," from late 15c. The classical literal sense of "left as opposed to right, in the left side (of the body)" is attested in English from c. 1500. In heraldry (from 1560s) sinister indicates "left, to the left." Related: Sinisterly; sinisterness.

Bend sinister (not bar sinister) in heraldry indicates illegitimacy and preserves the literal sense of "on or from the left side" (though in heraldry this is from the view of the bearer of the shield, not the observer of it; see bend (n.2)).