a. Suggesting or threatening harm or evil: a sinister smile. b. Causing or intending harm or evil; wicked: a sinister conspiracy. a. Portending misfortune or disaster; ominous: sinister storm clouds. b. Attended by or causing misfortune or disaster: "The day has passed without any sinister accident" (John Quincy Adams). a. Archaic On the left side; left. b. Heraldry Situated on or being the side of a shield on the wearer's left and the observer's right. [Middle English sinistre, unfavorable, from Old French, from Latin sinister, on the left, unlucky.] The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved. Indo-European & Semitic Roots AppendicesThousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.