Being full of shit. A know it all; a loser; a dullard, someone who isn't very bright. Could also mean someone is being an asshole. Having a load (of Shit) in your pants, Hence being full of shit. From an episode of King of the Hill where Bill steals a tank while intoxicated after learning that the Army experimented on him causing him severe weight gain and baldness... Bill, "Forgive me guys I was such a pant-load!!"
"Man did you hear what the President said?" "Yeah that guy is a total Pant-load!