"You're full of s---" is a ridiculous statement on a factual level. Why?

 Because everyone is "full of S---". Why?

Because if you are eating food it all turns to feces in your gut. So, if you aren't full of s--- then you are likely dead or dying.

I suppose you could fast like Jesus and Buddha both did for 40 days and maybe then you would have no food at all in your guts and then you wouldn't be "full of s---".