If Severus hadn't been entirely familiar with another student about Jake's age who had been trying to save the world, he probably wouldn't have believed him at first. But he was quite familiar with the Boy Who Lived, and so he did. He'd heard of New York, of course, and Kansas, but ... "Mid-World?" he asked, eyebrows rising.
He wasn't especially familiar with the term gunslinger, apart from calling to mind visions of the old American West. On the other hand, if 1977 and 1999 were any indication, he didn't think that was what Jake had meant at all. Well, perhaps in Mid-World, wherever that was -- probably Oy's home, come to think of it. "And what do you mean by gunslinger ... apart from the obvious, that is?"