Fishwife

A fishwife, fish-fag or fishlass is a woman who sells fish. She is typically the wife of a fisherman, selling her husband's catch, but other sources of fish have been used.

Some wives and daughters of fishermen were notoriously loud and foul-mouthed, as noted in the expression to swear like a fishwife, as they sold fish in the marketplace. Among the reasons for their outspokenness were that their wares were highly perishable and lost value if not sold quickly, and the similarity of their product to that of others selling the same thing, with volume of voice or colourful language drawing customer attention. Also, managing alone while their menfolk were away fishing for extended periods made them strong and self-sufficient.

In this context, the word wife means woman rather than married woman, from the Old English wif (woman).

Billingsgate

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."

In the 18th century, fishwives frequently appeared in satires as fearsome scourges of fops and foreigners.