the joke's on someone
idiomPart ofjoke
the joke's on someone — idiom
1. used to say that a person who tried to make another person look silly now appear
1.idiom
Definition
used to say that a person who tried to make another person look silly now appears silly or is at a disadvantage instead.
Example
Élise tried to trick her brother, but the joke was on her when he turned the tables.
Ilan planned to embarrass the speaker, but the joke was on him when the microphone picked up his whisper.