a dinni ken
A dinni ken is when you're too dumb to figure it out and you just say it like you're proud of yourself.
'I dinni ken why the bus is late.', said by someone who misses the bus every day.
'I dinni ken why I failed math.', said by a kid who still thinks 2 + 2 is 3.
'I dinni ken why I'm in trouble.', said by a kid who was caught eating candy during class.