Actually, the "Wild Bill" moniker was prevalent before Hickok became famous, so take it as you will. I've heard he was given the name while he was a stage driver, while he was fighting Kansas "Border Ruffians", and while he was acting as a scout for the Union during the Civil War. Actually, this is how legends are born; too many stories and not enough solid evidence, and there are plenty of stories regarding the "Prince of Pistoleers". I recommend "They Called Him Wild Bill" by Joseph G. Rosa as an excellent place to start seperating fact from fiction. Rosa is a Hickok fanatic, and much of his research was derived from the Hickok family, who opened up their archives to him.
My favorite Wild Bill story actually happened after his death. Seems that Deadwood was opening up a new cemetery (Mount Moriah) several years after W.B.s death, and his Pard, Charlie Utter, bought a plot to move him to. When they dug him up, however, the coffin mysteriously weighed a "ton", and it took fifteen men to raise it from the grave. Charlie said that W.B. had weighed 180 pounds at his funeral, but somehow mineralized ground water had seeped into the coffin, and had petrified W.B. so that he weighed somewhere closer to 490 pounds. They opened the coffin, and W.B. looked just as he did on the day of the funeral, with no sign of decay. Charlie reported that the skin was hard as a rock and sounded like wood when tapped; that his moustache had hardened, but his hair was just as shiny and pliable as ever (one person in the crowd even cut off a lock of hair as a souvenir). I've always wondered what he would look like if he was exhumed today. Still the same?