saying

[malcolms mum]

what or where does the saying come from /meam. Shes the cats mother?

[New Forester]

I think it all comes from the use of "a she" as the term for a female cat -- presumably originally short for she-cat.

 

I've heard your phrase used as a way of objecting to using a personal pronoun instead of someone's name, especially when they are present.

 

For example, I might say to my partner, talking of her mother, "Is she eating with us tonight?".  Her mum, overhearing, might say sharply "Who's she? The cat's mother?!".  She is suggesting that it would have been more respectful to use her name.