"Full of shit." For the same reason "porous" means "full of holes," "devious" means "full of deviance" etc.
Stupid Dude: Yo dudez! I heard there's like this fuckin crazy old lady at that run-down house, we should go check it out!!
Cool Dude: No way man, you're shitous.
The Urban Dictionary Mug
One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.
Buy the mug