Meaning of “the joke is on sb” in the English Dictionary

british dictionary

"the joke is on sb" in British English

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the joke is on sb

informal

If you say that the joke is on a particular person, you mean that that person has tried to make someone else look silly but has made himself or herself look silly instead.

(Definition of “the joke is on sb” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)