It originated in the BCA rule book. I'm not talking about BCA league, I mean the rule book that was always referred to before all these leagues with different rules. I have not read one of the recent issues, so it may have been changed, but the chalk width measurement was talked about in all the releases that I have read.
Edit: for my own curiosity I looked up the current BCA rules on their website. They have changed the wording of the rule and it no longer uses chalk as an example of width. It was in the older books and that is where the term came from.
I don't play in BCA (yet). I was introduced to pool via APA and that's where I've stayed. If "a chalk width" is used in BCA then that's where that rule should stay.
I've heard this brought up before and when I show them the bylaws they feign shock or let out the famous "well, that's not what I was told".
This thread is about APA and double hits and I can speak on that. BCA? Not so much.