say it: uhn-FAYTH-fuhl
unfaithful is an adjective meaning "not true to duty or obligation or promises". It carries 4 recorded senses.
having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend
“her husband was unfaithful”
having the character of, or characteristic of, a traitor
“the faithless Benedict Arnold”
“a lying traitorous insurrectionist”
Definitions, synonyms and semantic relations for unfaithful come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.