Is a person's saying something bad to another about his mother, despite the recipient's not knowing her beforehand, considered backbiting?

Informing someone that their mother committed a bad deed is considered backbiting (gheeba) and an act of disobedience (ʿuquq) towards the mother, even if the listener does not know her. This is because by specifying that it was his mother, she becomes identifiable. Backbiting about an unknown person is not permissible if it leads to the possibility of identifying them. However, general backbiting about an unknown person, such as "some people say such and such," is permissible as long as the listener does not understand who is being referred to.