sinister

Etymology

Recorded from the early 15th century;^[1][2] the OED's earliest quotation is from 1411.^[3] From Middle English sinistre (“unfavourable, unlucky”),^[4][5] borrowed from Old French senestre, sinistre “left; contrary, unfavourable”,^[1][3] from Latin sinister (“on the left side; unlucky, inauspicious”),^[2][4] a word of uncertain origin.^[1]

The “unlucky” sense comes from augury: omens, especially bird flights, seen on the left were taken to portend misfortune.^[1] Compare dexter.^[1]

References

1. [1] [2] [3] [4] [5] Harper, Douglas, Online Etymology Dictionary, https://www.etymonline.com
2. [1] [2] Merriam-Webster's Collegiate Dictionary, Springfield, Mass.: Merriam-Webster
3. [1] [2] Simpson, J. A.; Weiner, E. S. C., eds. (1989), The Oxford English Dictionary, 2nd ed., Oxford: Clarendon Press
4. [1] [2] The American Heritage Dictionary of the English Language, Boston: Houghton Mifflin
5. [1] Wiktionary, the free dictionary, https://en.wiktionary.org