A 'Cold joke' is a joke that's not funny at all, but the fact that somebody would tell such a joke is the real punch line.

冷笑话就是一点儿也不搞笑的笑话；其真正的笑点，在于竟然有人讲这样的笑话。

youdao

Because he was serious, and did not joke with his neighbours, people thought he was cold But when he loved or hated, it was with his whole heart.

他严肃，不和邻居们开玩笑，所以人们都觉得他很冷漠。可他爱谁或是恨谁时，却是全心全意的。

youdao