A fishwife or fish fag is a woman who sells fish. Such women were notoriously loud and foul-mouthed as in the proverb, To swear like a fishwife. One reason for their outspokenness is that their wares were highly perishable and so lost value if not sold quickly.
London's traditional fish market was frequented by such types who were known as "the wives of Billingsgate". "They dressed in strong 'stuff' gowns and quilted petticoats; their hair, caps and bonnets were flattened into one indistinguishable mass upon their heads. ... They smoked small pipes of tobacco, took snuff, drank gin and were known for their colourful language.