This noun typically refers to someone who claims that they can execute a difficult physical stunt, when in reality, they cannot, and are merely seeking props from fellow activity partners.
Raul: "Yeah? Show them to me right now."
Fuji: "Well, my ankle's kinda sore, so I can maybe do half a backflip right now."
Raul: "You're full of shig."
Fuji talks a lot of shig.