Probably been asked a hundered times, but here goes.
Wow, what is with my spelling. Sorry guys.
It probably has zero chance of being correct, but my dad told me when I was a little kid that it had to do with the Ph scale and was an indicator of its acidity (or lack thereof). Upper 10 was another lemon-lime drink in the South that had similar naming issues.
I’m curious to find if others have better explanations.
Because “Bib-Label Lithiated Lemon-Lime Soda” just didn’t cut it, and it had 7 flavors blended into it. From Cadbury-Schweppes history pages:
From http://www.cadburyschweppes.com/EN/Brands/About/Beverages/factsheet_sevenup.htm
…
OK, I didn’t go all the way to the bottom of the page.
BTW, “What is Bib-Label Lithiated blah blah” is a standard trivia contest question. As soon as you hear that much, you can answer “7 Up”. They took the lithium citrate out in 1950, as it turns out.
Snopes has a good page on it, too, confirming the “undetermined” status: