The cat’s mother
Q From Jane Van de Ban; a similar question came from Richard Dury in Italy: Can you tell me anything about the expression Who’s she? The cat’s mother? I’ve heard it used in a context in which you’re talking about a woman and referring to her as she rather than by name.
A How it came into being, I really can’t begin to discover. All I can tell you is that it’s first recorded about the end of the nineteenth century (at least, the Oxford English Dictionary has citations from that period; Jonathon Green says in his Cassell Dictionary of Slang that it dates only from the 1950s in the form of a direct reply to somebody asking rudely or intrusively “who are you?”). In its older form, as you say, it was usually said to a child who used she to refer to some grown-up when this was thought to be insufficiently polite.