鼠辈 is a classical/literary insult, common in historical dramas and wuxia (martial arts) fiction. The image of rats/mice (
鼠) suggests cowardly, sneaky, and despicable people.
辈 means 'generation/kind of person,' making
鼠辈 literally 'rat-kind' or 'the rat breed.' It sounds more theatrical than genuinely offensive in modern speech.
