What does he mean by "your balls drop off"?
Native speaker’s answer
Rebecca
Good question! The "balls" (testicles) are a male organ. By asking if the men's balls have dropped off, Joker is insulting them and implying they lack masculinity and bravery compared to a year before. The opposite expression, to "have balls," means to be brave or to have guts. These expressions are quite crude and informal, so it's not advisable to use them casually with just anyone. Additionally, they also play on gender stereotypes (i.e. that men must be masculine, strong, and brave).