I've never heard this phrase used in American English. It may be British English.

“Foul” means bad, angry, unpleasant, etc (in other contexts, it can mean bad smelling, against the rules, or out of bounds). A “foul tongue” refers to verbal abuse of some kind. “To lay on” doesn’t really mean much (I imagine you are familiar with its literal meaning), but is just there because all clauses require a verb in English.