Definition & Meaning of "the joke [is] on {sb}"

What is the origin of the idiom "the joke is on someone" and when to use it?

The origin of the idiom "the joke is on someone" can be traced back to the concept of humor and pranks. Humor often revolves around the unexpected or ironic, where the outcome is contrary to what was anticipated. In this idiom, "the joke" refers to a humorous or clever action taken by one person, while "on someone" implies that the humor or cleverness of the situation is at the expense of another person. It is used to emphasize situations where someone's efforts to be clever or humorous have unintended consequences.