I was watching an American show called Breaking Bad and they use this phrase:
Can I use your John’s?
Can I use your toilet?
As it stands the origin has been mentioned here, courtesy to @Jason Bassford and @user067531 in the comments below, but I feel it doesn’t adequately discuss why the particular name John is used, only on a probability that it is linked to the archaic euphemism of “Jack’s” to mean toilets.
What I’m asking goes deeper: it is why Jack or John have been used as euphemisms to refer to a toilet in particular?
Why Jack or John as euphemisms, can toilets only be euphemistically named after names beginning with J?
Also for clarification, several sources on its etymology say different things and it’s mostly speculation. Since that linked “similar” question was asked in 2011, I was hoping for more insightful answers. Perhaps modern usage of these phrases has also changed over time, and hence its meaning, so it will be interesting to learn about those too, if any.