One who contracts to deliver a certain quantity or sum of stock in the public funds, on a future day, and at stated price; or, in other words, sells what he has not got, like the huntsman in the fable, who sold the bear’s skin before the bear was killed. As the bear sells the stock he is not possessed of, so the bull purchases what he has not money to pay for; but in case of any alteration in the price agreed on, either party pays or receives the difference. Exchange Alley.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Bean * Bear-garden JawNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes