Watson, of course, mentioned to Holmes at their first meeting - while they're fessing up about personal quirks that might affect them as roommates - that he "keeps a bull pup". Many readers have wondered what happened to the dog, as it never appears in later stories.
This is because there was no dog: what would a recently discharged army doctor, just back from Afghanistan, and living in a rooming house, be doing with a dog?
Watson was really saying that he kept a revolver.
I like old revolvers, and the Webley company manufactured many of them in that era: hence, I put the two words together, and Webley Bullpup I became. :-)