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. ["Dictionary of the Vulgar Tongue," 1811]
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 is mid-13c., not exclusively a fish market until late 17c.[billingsgate etymology, billingsgate origin, 英语词源]