a. Not true to duty or obligation; disloyal or unfaithful.
b. Breaking trust in a marriage or relationship by having sexual relations with someone other than one's spouse or sexual partner.
2. Having no religious faith.
Synonyms: faithless, unfaithful, false, disloyal, traitorous, perfidious
These adjectives mean not true to duty or obligation. Faithless and unfaithful imply failure to adhere to promises, obligations, or allegiances: was faithless to her ideals; an unfaithful servant.
False emphasizes deceitfulness or duplicity: "To thine own self be true, / And it must follow, as the night the day, / Thou canst not then be false to any man" (Shakespeare).
One who is disloyal betrays an allegiance: disloyal staff members who exposed the senator's indiscretions.
Traitorous most commonly refers to disloyalty to a government or nation: a traitorous double agent.
Perfidious often connotes vile or contemptible behavior: "The propagandists also highlighted atrocities committed by enemy soldiers, in order to demonstrate the perfidious qualities of the enemy nation" (David A. Bell).
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 Appendices
Thousands 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.