There are many different ways to describe someone who frequently uses foul language. Some synonyms for the phrase "was foul-mouthed" might include "was vulgar," "had a potty mouth," "swore like a sailor," "used profanity often," or "had a filthy tongue." Depending on the context and the severity of the person's language, other synonyms might include "was obscene," "used obscenities frequently," "had a nasty habit of swearing," or "was crude in conversation." Regardless of the specific wording used, it is clear that someone who is frequently foul-mouthed is not known for their polite, refined language habits.