Thank you so much @Oreosareawful, I think that you might have just solved a puzzle for me that I have wanted the answer to for about 58 years! Even though with the Internet we can look up almost anything these days, even Google couldn't have answered this question for me.
I must have been about 6 or 7 years old, and I was shopping with my dear Mum for some clothes for me. She used to like getting me pretty dresses from a shop called "Ladybird/s", the shop was in St Albans, which was quite near the town we lived in - it was quite a posh shop so I don't know if it was the same brand as the ladybird brand that Woolworths went on to stock some years later (if it was then the quality of clothes went down quite a lot in the interim) well according to my lovely Mum anyway!
It was almost a full day's outing for Mum and me as she didn't drive so we used to go on a bus to get there (where possible we would sit upstairs, and right at the front, as I used to love pretending I was driving the bus), and because we were out all day we would have lunch in our favourite restaurant - it was a proper restaurant with white tablecloths and a waitress service, the waitresses wore black dresses with white aprons (isn't it weird the strange things we suddenly remember). Even though it was a "proper restaurant" they seemed happy to serve me with my favourite lunch - Heinz tinned spaghetti on toast (they did take the spaghetti out of the tin and heat it up for me first!), and a glass of lemonade (the only time I was normally allowed to drink lemonade was on my birthday and at Christmas, so it really was a lovely treat for me.
At last I am getting to the point I have been trying to reach, in order to explain to you about how your son's word mix-up has satisfied me that I have the right answer at last to my frustrating, over half a century's year old, question.
I burped after drinking my lemonade, so I said sorry for the burp and I told my mum that I had fizz up my nose from the burp. My Mum gave me a very rare telling off, and said that I must never use that word again. I was very baffled by that, and I wished I had questioned her at the time, but I somehow thought that my Mum might think I was being rude by questioning her reason for the ban on the word fizz. Over the years I have sometimes gone back to wondering what was wrong with me saying the word fizz, I even asked my Mum about it once many years later, but she couldn't remember the incident at all.
If you are somehow still reading this @Oreosareawful - or anyone else for that matter 🤭 - I think you have probably guessed by now what I think my Mum thought I had said, ie that I had "piss up my nose from my burp"! I guess she must have thought I had picked up the word from school, as it wasn't one ever used at home. I find it quite amusing now that I wouldn't have had any idea about what the word piss meant in those days 😂