we use this word(설마), when we have a little doubt, or we think it wouldn't happen. for example, 1. 설마 ~하겠어? 할까~? 설마 걔가 백점맞겠어? (I don't think so(설마-하겠어?) he can get a perfect score) 2. or, A: He is already gone to Africa. B: oh,really? 정말? you're kidding(설마 or 설마 그럴리가). It is little difficult to explain how to use it.

I also watch korean drama, and heard this words many times. I think the meaning is kind of "guess" which someone is shocked to believe it. For example: 설마 you love her?