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.

Definitions of unfaithful
  1. adjective
    not true to duty or obligation or promises
    “an unfaithful lover”
    likely to change frequently often without apparent or cogent reason; variable
    not faithful to religion or party or cause
    perfidious, punic, treacherous
    tending to betray; especially having a treacherous character as attributed to the Carthaginians by the Romans
    not true to an obligation or trust
    untrustworthy, untrusty
    not worthy of trust or belief
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    steadfast in affection or allegiance
    steadfast in purpose or devotion or affection
    fast, firm, loyal, truehearted
    unwavering in devotion to friend or vow or cause
    devoted (sometimes fanatically) to a cause or concept or truth
    consistent with fact or reality; not false
    trustworthy, trusty
    worthy of trust or belief
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  2. adjective
    having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend
    “her husband was unfaithful
    adulterous, cheating, two-timing
    not faithful to a spouse or lover
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    not having sexual relations with anyone except your husband or wife, or your boyfriend or girlfriend
    true to
    sexually faithful
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  3. adjective
    having the character of, or characteristic of, a traitor
    synonyms: faithless, traitorous, treasonable, treasonous
    deserting your allegiance or duty to leader or cause or principle
  4. adjective
    not trustworthy
    “an unfaithful reproduction”
    not exact
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