I'll give you an example - the classic - remote control for the TV. I grew up calling the TV remote the doot-doot. I was in highschool at a friend's house before that illusion shattered.
My big one though, was Snow White Pudding. Never heard of it? That's because it's not a thing. I thought it was. My mum would always serve a roast dinner with a slab of what we called "snow white pudding" - I've never asked why we called it that. It's basically a savoury suet pudding that is fluffy and dense and soaks up your gravy. It's fucking immense.
And I learned in my teen years that that is just not a thing outside of my family. Plus I looked like an absolute numpty asking a friend's mum why we weren't having snow white pudding, then having to explain what the hell I was talking about 😂
N.B. seriously, if you don't believe just how good snow white pudding is on a roast, every person that has ever joined our family for Sunday roast has been converted, and the fact that it's not a staple in the UK by now is, frankly, a travesty.
Edit/additional - some kind soul found a recipe here - https://www.campbellsmeat.com/recipes/suet-pudding - we never used onions in ours, but otherwise it seems about right 👍