E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Gate = quay, and bellan is to bawl or bellow. This quay is so called from the shouting of the fishermen in trying to attract attention and vend their fish.
Thats Billingsgate. Vulgar and coarse, like the manners and language of Billingsgate fish-fags.
Parnassus spoke the cant of Billingsgate.
Dryden: Art of Poetry, c. 1.
To talk Billingsgate, i.e. to slang, to scold in a vulgar, coarse style.
You are no better than a Billingsgate fish-fag, i.e. you are as rude and ill-mannered as the women of Billingsgate fish-market. The French say Maubert instead of Billingsgate, as Your compliments are like those of the Place Maubert, i.e. no compliments at all, but vulgar dirt-flinging. The Place Maubert has long been noted for its market.