Bull
Bull (and its antonym bear ) became slang for market optimists (and pessimists) during the last century. The terms may have been derived from the bullrings and bear pits in Southwark which were frequented by traders after work.
In 1894 Charles Henry Dow defined a bull market as one which rose for at least two years. In the decades following the Second World War there was quite a strict definition of a bull in the London market.
It was somebody who bought shares at the beginning of the stockmarket's then two-week trading account with the aim of selling them for a profit before the end of the account.
Expecting the market to rise, the bull would hope to pocket the gain in the share price and sell his shares before having to pay for them.
Now that the London market has moved towards rolling settlement and has already shortened its settlement times considerably, a bull is taken to mean anybody who believes share prices will rise and continue to rise. A bull market is one in which share prices are generally rising.