Definition: coarsely abusive, foul, or profane language.

Quotation: “Billingsgate most foul issued forth from the one known as John Los upon espying Tyler of Hansbrough’s dunk on Hibbertroll’s head.”

Alright, maybe that didn’t come from a respected piece of literature, but I think it clearly elucidates the word’s meaning. Interestingly, I found a nice bit of trivia when I was looking for a good quote. This from What I Saw in London: Or, Men and Things in the Great Metropolis by David W. Bartlett (I think he wrote another book I quoted from earlier)…
“Foul language is often characterized as ‘billingsgate,’ but we dare say there are some in America who use the word without understanding its origin. It comes from the great fish-market in London, called Billingsgate. It is the only whole sale fish-market in London, and necessarily is a scene of great confusion. The people who do business in it are a low and dirty class, and at all hours of the day the market is the scene of noise, confusion, and filth. By degrees, the people got to calling language which was foul and noisy, ‘billingsgate.’”