unfaithful

American  

[uhn-feyth-fuhl] / ʌnˈfeɪθ fəl /

adjective

1. 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

2. not sexually loyal to a spouse or lover; adulterous.

   She was concerned her husband would be unfaithful when he travelled.

3. not accurate or complete; inexact.

   The book was an unfaithful translation of the original.

   Synonyms:

   untrue, imprecise

4. Obsolete. without religious faith; unbelieving.

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unfaithful British  

/ ʌnˈfeɪθfʊl /

adjective

1. not true to a promise, vow, etc

2. not true to a wife, husband, lover, etc, esp in having sexual intercourse with someone else

3. inaccurate; inexact; unreliable; untrustworthy

   unfaithful copy

4. obsolete not having religious faith; infidel

5. obsolete not upright; dishonest

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Other Word Forms

• unfaithfully adverb
• unfaithfulness noun

Etymology

Origin of unfaithful

First recorded in 1350–1400; Middle English unfeithful, unfaithful; un- ¹ + faithful ( def. )

Example Sentences

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

"And three months ago, I made the biggest mistake of my life and I was unfaithful," he continued.

From Barron's • Feb. 11, 2026

According to Pratt, MTV was not satisfied with him merely portraying an unfaithful boyfriend.

From Los Angeles Times • Jan. 27, 2026

They divorced after he admitted to being unfaithful during the marriage.

From BBC • Sep. 10, 2024

He knew that Mae was unfaithful to him.

From "A Tree Grows in Brooklyn" by Betty Smith

Related Words

• cheating
• untrue