Emo Handshake

Listen up, buttercup. The joke is about an "emo kid," right? Emo kids are known to be sad and angsty. So, when the person says they "left him hanging," it's a double whammy. They physically didn't shake the emo kid's hand, and they also metaphorically left him hanging, which is like ditching him in his sadness. I know you probably think hanging out means getting ice cream, but get with the program, kiddo.