The term originates from one of the key theologies in the book, the Seven, of which there are seven deities, with seven heavens, and, therefore seven hells.
The term is often used as an expression of anger, anguish, or surprise, but it not limited to these emotions.
This term should be considered akin to the word "fuck" in its usage, however, it is usually used by a higher-class person, such as the royalty in the book.
Robb: "Seven hells! Where did they come from?" (surprise)
Robert: "Seven bloody hells, Ned, I want to HIT something!" (anger)
Catelyn: "Seven hells! Why can't you see that this won't solve anything?" (anguish)
Cersei: "Seven hells, Jamie, why won't you ever think?" (anger)