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, watchingfootball, 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