What does "proper old bag" mean? Is Knuckle calling Aunt Lucy a "has-been"? Or more aggressively calling her a "boomer" perhaps? And is it a common expression?

Native speaker’s answer
Rebecca
In the UK, "old bag" is a rather derogatory slang expression used to describe a woman who is ill-tempered, disagreeable, or unpleasant. It's more commonly used to refer to older women, but the speaker has used it here not to emphasize Aunt Lucy's age but to express his negative impression of her (probably because he finds her unpleasant). "Proper" here is used to mean "real" or "very." Ex: The lady who lives next door to me is an old bag. She complains about everything. Ex: Someone called me an old bag this morning just because I complained about them cutting in line at the store.