Similar to HitStick popularized by Madden 2005 (and since). In the game when someone is HitStick they are bewildered and shaken from the event. ShitStick is farting around someone and having the same effect.
Matt: What is that?!
Jay: I definetaly reefed a fart at you...
Matt: Dude that sounded like a low flying jet.
Jay: It's my new ShitStick...enjoy.