unfaithful
Meaning
not loyal to a sexual partner by having a secret relationship with someone else
not loyal to a friend, country, belief, etc.; not deserving trust
Structure
Usage
she decided to leave her unfaithful husband.
the minister was accused of being unfaithful to the party's ideals.
a biography that is unfaithful to the historical facts is misleading.
Etymology
the word unfaithful is a layered english construction. its core, "faith," entered english from old french "feid," tracing back to latin "fides," meaning trust or belief. the latin term itself is rooted in the proto-indo-european stem bʰeydʰ-, meaning "to trust, confide, persuade." the old english suffix "-ful," meaning "full of," was attached to nouns to create adjectives describing a quality. finally, the old english negative prefix "un-" was applied, directly inverting the meaning. thus, the literal "not full of trust" evolved into the modern abstract senses of disloyalty, whether in personal relationships or to principles.
Analysis
un: a prefix of germanic origin (old english) meaning "not". faith: a noun of latin origin (via old french), from latin 'fides' meaning trust, belief. ful: a suffix of germanic origin (old english) meaning "full of" or "characterized by".