[By that she's going to assume Dawnie didn't say much about her to Sam]
I'm magic. Not like Glinda but...a long time ago some mygonistic old guys had a vampire problem that none of them wanted to fight so they hit a girl with some whammy that made her stringer, faster, more durable with an extra dose of super healing. She was the slayer. That gets passed down from generation to generation. I'm the slayer now; harder to kill than your average cockroach.

[She can get into the other slayers later] And ewww I just referred to myself as a cockroach. I might be dying.