Billingsgate,
a word said to have been derived from Belinus Magnus, a somewhat mythic British prince, father of King Lud, about B.C. 400. More probably it came from some unknown person called Billing. It is applied to the celebrated London fish market at least as early as A.D. 979, made a free market in 1699, extended in 1849, rebuilt in 1852, and finally exposed to the rivalry of another market begun 1874, completed 1876. The word is also used to indicate foul, abusive language, such as popularly supposed to be mutually employed by fishwives who are unable to come to an amicable understanding as to the proper price of the fish about which they are negotiating.
Entry from Everybody's Cyclopedia, 1912.