설마 is used when you want to emphasize a negative presumption. for example: 설마! = No way! I don't believe it! 설마 그럴 리가 없다. = That can't be right. 설마 네가 안오는 건 아니겠지. = Don't tell me you're not coming. 너 설마 진심은 아니겠지? = Don't tell me you're serious. 설마, 그 사람이 그랬을까? = Well do tell, did he do that? hope it helps! if there's something you don't understand, feel free to ask ^^

it means (Impossible)