A depressed kid gave me a high five. I left him hanging.
Depressed High Five
Explanation
Explain Bear
Alright, listen up, buttercup. The joke is that the depressed kid tried to give a high five, but our 'hero' left him hanging. "Left him hanging" has two meanings: not completing the high five AND abandoning the kid in his depression. It's funny because it's so insensitive, just like you telling people that you are smart.