I think the answer is on the previous thread
alphajuliet123 Tue 04-Dec-18 21:21:36
Picture the scene. Christmas 1993. Uncle Knobhead is a bit tipsy and regaling the fam with more hilarious tales. He starts another story with the words:
"There was a man whose name was Boswell, for short they called him Bos, andhewalkedupthepostofficesteps..."
And quick as a flash someone wittily intercepted with "that's not short for Boswell!!"
And how they all laughed. Auntie Janice peed her pants, mum dropped the sprouts and the Bos joke was immortalised for ever. It may well have been quite funny at the time but you probably had to be there.
I BET THIS IS EXACTLY HOW IT ALL HAPPENED!!
And it's been an in-joke forever more. The OP says the family are adamant that it could be a different gender and a different location and it doesn't have to be stairs etc, etc, etc. It's all about the delivery of the line.