unfaithful
Americanadjective
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not faithful; false to duty, obligation, or promises; disloyal.
Given how unfaithful the party has been to voters, it would be surprising if they won.
- Synonyms:
- recreant, treacherous, deceitful, untrustworthy
-
not sexually loyal to a spouse or lover; adulterous.
She was concerned her husband would be unfaithful when he travelled.
-
not accurate or complete; inexact.
The book was an unfaithful translation of the original.
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Obsolete. without religious faith; unbelieving.
adjective
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not true to a promise, vow, etc
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not true to a wife, husband, lover, etc, esp in having sexual intercourse with someone else
-
inaccurate; inexact; unreliable; untrustworthy
unfaithful copy
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obsolete not having religious faith; infidel
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obsolete not upright; dishonest
Other Word Forms
Etymology
Origin of unfaithful
First recorded in 1350–1400; Middle English unfeithful, unfaithful; un- 1 + faithful ( def. )
Explanation
When someone is unfaithful, they're disloyal or unreliable. If you don't honor the pledge you made "to help people at all times," you're being unfaithful to the Girl Scout code. While it's common to use the word unfaithful for people who break their marriage vows, it's a great way to describe someone who's broken any serious promise or neglected an obligation. An elected government official who doesn't keep their promises to voters is an unfaithful public servant. And if your brother refuses to participate in the family's annual Halloween custom of dressing as pirates, you can say he's being unfaithful to the tradition.
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
Diana wore the daring off-the-shoulder gown to a gala dinner in London in 1994 on the day that her husband, now the king, admitted during a TV interview to being unfaithful.
From Barron's ● Sep. 10, 2026
The unfaithful Clytemnestra and Helen are supposed to be contrasting forces to Anne Hathaway’s loyal Penelope.
From Salon ● Sep. 3, 2026
Then there’s the Norwegian biathlete who admitted to being unfaithful in a very public bid for forgiveness.
From The Wall Street Journal ● Feb. 13, 2026
According to Pratt, MTV was not satisfied with him merely portraying an unfaithful boyfriend.
From Los Angeles Times ● Jan. 27, 2026
He knew that Mae was unfaithful to him.
From "A Tree Grows in Brooklyn" by Betty Smith
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
