1. not faithful to a spouse or lover - adulterous husbands and wives - a two-timing boyfriend • Syn: adulterous, two-timing • Similar to: unfaithful • Derivationally related forms: adultery (for: adulterous) 2. violating accepted standards or rules - a dirty fighter - used foul means to gain power - a nasty unsporting serve - fined for unsportsmanlike behavior • Syn: dirty, foul, unsporting, unsportsmanlike • Similar to: unfair, unjust • Derivationally related forms: foulness (for: foul)