billingsgate n.

1670s, coarse, abusive language of the sort once used by women in the Billingsgate market on the River Thames below London Bridge.

Billingsgate is the market where the fishwomen assemble to purchase fish; and where, in their dealings and disputes they are somewhat apt to leave decency and good manners a little on the left hand. [Grose, "A Classical Dictionary of the Vulgar Tongue," 1788]

The place name is Old English Billingesgate, "gate of (a man called) Billing;" the "gate" probably being a gap in the Roman river wall. The market there is from mid-13c.; it was not exclusively a fish market until late 17c.