unfaithful

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

Synonyms of unfaithful

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

Examples of unfaithful in a Sentence

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

Recent Examples on the Web

In other words, some hallucinated or unfaithful answers originate before generation. — Patrick Dajos, Forbes.com, 23 Sep. 2026

Patrick testified and found himself at the center of an online swarm of sleuths and influencers dissecting every detail of the case, developing theories that portray him as uncaring, unfaithful and even the murderer of his children. — Ross Douthat, CBS News, 20 Sep. 2026

Many researchers were convinced that the group must also be unfaithful and fished around to find the relationship in Birman’s approach or Moody’s method. — Lyndie Chiou, Scientific American, 24 July 2026

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

Word History

First Known Use

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