- sinister (adj.)
- early 15c., "prompted by malice or ill-will," from Old French sinistre "contrary, unfavorable, to the left," from Latin sinister "left, on the left side" (opposite of dexter), perhaps from root *sen- and meaning properly "the slower or weaker hand" [Tucker], but Buck suggests it's a euphemism (see left (adj.)), connected with the root of Sanskrit saniyan "more useful, more advantageous."
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 left was favorable. Bend (not "bar") sinister in heraldry indicates illegitimacy and preserves the literal sense of "on the left side."