What is the origin of the idiom "not know one's ass from one's elbow" and when to use it?

The origin of the idiom "not know one's ass from one's elbow" is not exactly known. It is a vulgar and humorous way of saying that someone is completely clueless or ignorant.The phrase uses the anatomical terms "ass" (buttocks) and "elbow" to create a comically exaggerated image of someone who can't distinguish between two very different body parts, symbolizing a lack of basic knowledge or common sense. This idiomatic phrase is used to express strong disapproval or frustration with someone's incompetence or lack of understanding.