A regular person. Often but not necessarily with a slightly negative connotation. Etymologically related to "Average Joe" and "Schmo" a Yiddish word for idiot.
I'm scared as hell of becoming some Joe Schmo, watching football, drinking beer that my fat wife brings me, and blaming my farts on my dog.
Friend 1: I'll just drive her out to "get lucky rock" then once we're out there I'll just say I'm out of gas and that we'll have to stay the night out there
Friend 2: I don't know man thats about an 8.5 on my schemometer
Friend 1: What?
Friend 2: Obviously you schemed about that plan for a while, you're a creep