you don't know your ass from your elbow????

[giggs]

i listened to dane cook's stand- up comedy everytime, and, in the Benson's animals farm, his faher says' you don't know your ass from your elbow' and he says' i never went to the bathroom' what does it mean? how are these relevant to each other?
does it mean that he can't see the asses in the view of elbow?
does he personify the elbow?
*** means as..s