I love this question. I'm asking some of the old timers (who are no doubt younger than me) to get the backstory.
I also assumed it was a combination of "Site" as in "Web site" and "Infinity" as in "You can do anything with it". Beyond that, it's a product name that wasn't already taken...which is always a challenge with product naming.
I'll wait for a reply on this and post more details if I get them.