I googled around about this last night since I’ve never heard this expression, and the best I could come up with that sounded plausible is that a female cat is called a “she-cat,” so “she” might refer to “the cat’s mother.” It’s a little sketchy (I always thought female cats (especially mothers) were called "queens), but it’s the best I could come up with. Since it seemed to originate in the UK, maybe “she-cat” is a more British expression.
My husband grew up in south Texas and I grew up in middle Tennessee and we both remember getting in trouble for referring to our mothers as “she.” My husband also remembers getting in trouble for calling his mom “her” and I was grounded for calling my mom “your wife” when complaining about my mother to my dad. Of course, it probably had more to do with our tones at the time-we both admit we were being sassy.
I’ve been racking my brains for a specific memory of what I’d said to trigger the cat’s mother thing all I’ve got is a sense of indignation that it sometimes happened when I thought I was using the pronoun rule correctly like in the example above but the adults didn’t think so. We (my big brother and I) never took it too seriously just filed it under weird things they say.
I grew up in the '90s in Texas and it was disrespectful to call my stepmom “she” if she was in the same room. Not so much if she was elsewhere and the context was clear. That was probably to enforce the idea that she was to be respected just as much as Dad, though. I thought it was stupid then but it makes sense now.
ETA: Nothing about cats, though.
I got a clip around the ear for calling my mother ‘she’. I think it was all in the tone, though. And cats were definitely mentioned.
I’m not from New York, or even the US, so this ‘cat’s mother’ thing is a widespread phenomenon.
Never heard of such a thing.
NYC born, Nassau County (LI) bred. Parents from Queens/Brooklyn. All granparents from Brooklyn/Bronx. Lived in NJ for the past 15 years. Don’t know any cats’ mothers.
My dad said this several times. He was a Depression kid from Appalachia. Come to think of it, he never referred to my mom as “Mom” or “your mom” – always “your mother.”
But I never heard of this “cat’s mother” thing.
Bacon Salt and sleeping have gotten to the crux of this issue in my understanding.
It is about referring to people in the third person when they are in your presence.
It is rude to do so, because you are not acknowledging their presence in the room.
It is particularly egregious (lord, this is never egregious) when the third person pronoun is at the beginning of the sentence or conversation.
If you have already acknowledged them by stating their name or title earlier, and thereby recognizing that they are part of the conversation, then you are free to use the pronoun.
So if Mom is out of the room…you could point at a picture of her and say, “She sure is purty isn’t she?”
If Mom is in the room, you could say to your Dad, “Mom sure is purty, today, isn’t she? She put on a brand new dress.”
Never heard the thing about the cat.
Chicago.
Posting again to add that this is true not just of Moms, but of anyone deserving of politeness.
However, as this thread demonstrates, this “rule” is certainly not universally known, and is pretty esoteric in any case.
There have been one or two times when I’ve slipped up, and then turned to the “slighted” person and said, “Oh, I’m sorry, I just referred to you in the third person.”
They responded with a confused look and a, “Huh?”.
I’m absolutely fascinated by this thread.
My mother is from Poland, and she pulled the exact same crap on me. As far as I’m able to understand, in the Polish culture at least, the use of a pronoun to refer to a close family member is considered … not exactly rude, perhaps, but … cold? Impersonal? Something more along those lines.
This would be in line with the posters talking about relatives from different backgrounds, I guess.
I chalked it up to a) cultural differences, and b) my mother is batshit crazy.
This is fascinating to me as well. I grew up in the western edge of the midwest, raised by parents who were third and fourth generation in that state, so no Polish enclaves for anybody. 
We were never reprimanded for using the third person pronoun unless we had not made the subject clear in the conversation.
There were definitely no references to cats. That line cracks me up.
I grew up in New England (MA) and never heard of it. And she… I mean, my Mom… grew up in New Jersey.
I got told this all the time growing up. Until, one day, I just hauled off and shouted, “it’s a pronoun! It’s clearer that way!”
This from my Dad, btw; I don’t think my mom cared one whit. And, honest to God, I just thought it was my dad being a dick. Apparently not.
I’m from the Chicago area, by the way. My dad’s dad was from South Dakota, and his mom’s parents were from Greece.
So let me see if I understand this. Instead of being able to say “Mom said she thinks she forgot that she went to the store already,” you would have to say something like “Mom said Mom thinks Mom forgot that Mom went to the store already”? That just seems silly to me.
Native Chicagoan. Never heard this one.
Interesting. I grew up in the Philly 'burbs (Norristown) and I don’t recall ever hearing this.
The rule was never enforced in our house (in California), but I was aware of it and “the cat’s mother” from childhood reading. I always assumed the meaning was that your mother is entitled to respect, so don’t refer to her like she’s a household animal.
I don’t know if I’d ever heard the phrase spoken aloud though until an episode of “Doctor Who” (second season of the new series, I think).
How is she clearer than Mom?
Of course not. One Mom is enough – on so many levels.
First of all, that was not me with the ices.
Anyway, I’m from the South, not the Yankee states, but no, you’re not supposed to call your mother “she”. My mother doesn’t even like it when her husband calls her “she”.
Most of my friends, the ones I grew up with, now call their fathers by first names. It’s a kind of transition from subject to peer.
My English grandmother once scolded me as a child because, while relaying a conversation between her and someone our of her earshot (but within mine), I said “She (grandma) said that you need to…”.