You're right, I hadn't thought about it. It's true it's quite used, although, if we had to compare it to English or Portuguese, it's a lot less common. Apparently, it comes from "señor" through continuous erosion.@Dymn People in the Spanish heartland say for sure: ¡So imbécil! ¡So cabrón!
But so is not a word as far as I know, so it's confusing, unless it comes from pedazo.