Definition of the joke is on someone in US English:

the joke is on someone

phrase

informal
  • Someone looks foolish, especially after trying to make someone else look so.

    • ‘They don't seem to realise that the joke is on them.’
    • ‘It would seem that the joke is on someone, but who?’
    • ‘All I can say to those five friends who started it all is, thanks lads and the joke is on you.’
    • ‘But I'm betting that the Butler has more to do with the outcome of the show than we think, and that the joke is on Joe.’
    • ‘But the joke is on you because I have backup right up stairs.’
    • ‘Or would you start to suspect that the joke is on you?’
    • ‘When it is clear how ridiculous and ludicrous our situation is, the joke is on us.’
    • ‘Everyone laughs, though we're unsure if the joke is on us.’
    • ‘On the other hand, perhaps the joke is on me for even considering this nonsense seriously.’
    • ‘He knows when to tease and always understands when the joke is on him.’