Its always going to be debated, but apparently some people have contacted subaru and X doesn't mean anything.
What does WRX stand for? WR stands for "World Rally", and "X" stands for the mathematical symbol for the unknown, that means its possibility is infinite. Many people also have other ideas what the "X" stands for, but this information comes from FHI/STi and was further confirmed by SOA.
I've also heard all of these at some point. A lot of them people say that it comes strait from Subaru but in reality no one probably will ever know. Another really good theory that I heard before the wrx even came here was they wanted to name if impreza wrc but obviously wasn't able to so they threw the x in there. That once was also "confirmed" by Subaru back then. I just think pick your favorite and stick with it.