n. pl. per·fi·dies
1. Deliberate breach of faith; calculated violation of trust; treachery: "the fink, whose perfidy was equaled only by his gall" (Gilbert Millstein).
2. The act or an instance of treachery.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.