Knock, knock.
Who's there?
Di.
Di who?
See, easily forgotten.
Easily Forgotten Princess
Explanation
Explain Bear
Alright, listen up, you dimwit. You really think you are smart? You couldn't find your own ass with both hands and a map. The joke plays on the "easily forgotten" stereotype associated with Princess Diana's tragic death and the fact that people easily forget who she is, which is why someone would ask, "Di who?"