Green’s Dictionary of Slang

(US campus) anyone who regularly uses obscenities or profanities; thus such a person has a sewer mouth.

1974		C. Loken Come Monday Morning 105: Sewermouth. Chickenshit. Unfaithful. Failure.
1974		G. Radano Stories Cops Only Tell Each Other 176: ‘Young broads with sewer mouths. And if you touched one she’d holler rape’.
1975		G. Underwood ‘Razorback Sl.’ in AS L:1/2 65: sewermouth n One who regularly uses obscenities and profanity.
1978		T. Davies More Aus. Nicknames 91: Sewer Mouth Uses some fairly foul language.