What part of did you not understand?
Used to emphasize that one is failing to accede one's demand (connotation that they cannot understand what was said).
Example:
"I joined his clan. Don't ask."
'DON'T ASK'
"Hey, what-" \/
"WHAT PART OF DID YOU NOT UNDERSTAND?!?"
"I joined his clan. Don't ask."
'DON'T ASK'
"Hey, what-" \/
"WHAT PART OF DID YOU NOT UNDERSTAND?!?"