"The name “Bull Durham” is said to have been taken from the bull on the British Coleman Mustard, which Mr. Blackwell mistakenly believed was manufactured in Durham England. From 1874 – 1957, Bull Durham Tobacco, the first truly national tobacco brand, was manufactured in Durham, NC."

The Durham Bulls are a minor league baseball team in Durham. If you drive from I-40 to I-85 on the Durham Expressway, you'll pass the ball field on your right.

In this case, he's referring to John Durham, how he looks, what he's doing, and that most people have heard of the movie, making for a catchy term.