Swear to God

As we are extremely polite and well-educated people, we won't show what he really said, but we think that you can imagine, can't you?
In English, "swear" has these two meanings: one is the same as "promise", while the other means "curse". So, what the girl meant to say was "promise in the name of God", while the boy understood that "Swear to God" would have the same meaning as "Curse God".