unfaithful

1 of 2

adjective

un·​faith·​ful ˌən-ˈfāth-fəl 

: not faithful:

a

: not adhering to vows, allegiance, or duty : disloyal

an unfaithful friend

b

: not faithful to marriage vows

suspected her husband of being unfaithful

c

: inaccurate, untrustworthy

an unfaithful copy of a document

unfaithfully

ˌən-ˈfāth-fə-lē 

adverb

unfaithfulness noun

unfaithfulness

2 of 2

noun

un·​faithfulness

"+

: the quality or state of being unfaithful

Synonyms of unfaithful

Adjective

• traitorous
• unreliable
• false
• faithless
• disloyal
• treacherous

Examples of unfaithful in a Sentence

Adjective

colonists who later proved to be unfaithful to the cause of independence

Recent Examples on the Web

Adjective

After so many years as the good guy, Klein is leaning into his dark side, taking on roles like the big bad Cicada on CW’s The Flash, and as JoAnna Garcia Swisher’s unfaithful ex-husband on the Netflix series Sweet Magnolias. — Gwen Ihnat, Entertainment Weekly, 9 July 2026

Nyong’o also plays Clytemnestra, the unfaithful wife of Agamemnon, whom Clytemnestra and her lover, Aegisthus, murder. — Encyclopedia Britannica, 1 July 2026

In Strangers on a Train, Rogers played Guy's unfaithful wife, whom Bruno offers to kill. — Emily Blackwood, PEOPLE, 29 June 2026

Platner is also an unfaithful spouse who mistreats women. — Chris Roemer, Baltimore Sun, 25 June 2026

Word History

Etymology

Noun

Middle English unfeithfulnesse, from unfeithful + -nesse -ness

First Known Use

Adjective

15th century, in the meaning defined above

Rhymes for unfaithful

• faithful

Kids Definition

unfaithful

adjective

un·​faith·​ful ˌən-ˈfāth-fəl 

ˈən-

1

: not observing vows, allegiance, or duty : disloyal

an unfaithful friend

2

: not faithful to marriage vows

3

: inaccurate

an unfaithful translation

unfaithfully

-fə-lē 

adverb

unfaithfulness noun