I heard a dialogue in a movie between a teenage daughter and her mom, which rummaged through the daughter's purse and found some condoms:
- Daughter: Tu as fouillé dans mon sac?
- Mother: On s'en fout de ton sac! C'est quoi ça?
I know that "se foutre de" means "to not give a damn", but why does the mother uses "on" and "en" here? "on" means "we" or an impersonal subject, but neither of them make sense here. "en" is a pronoun which means "de qqch", but the complement of "se foutre de" is explicitly mentioned in the sentence" de ton sac. Is "on se foutre de" a fixed expression?