The story
From the fact that the female protagonists in such novels would often have their bodices (blouses or corsets) removed with some force before sexual activity commenced.
Etymology from Wiktionary, CC BY-SA 4.0.
bodice ripper
bod·ice rip·perToday it means: A romantic novel, usually in a historical setting, with frank depictions of sexual activity, especially one in which the female protagonist is seduced; (by extension) a film, television programme, etc., featuring such activity. Full definition →