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.