the joke is on someone
US /ðə dʒoʊk ɪz ɑn ˈsʌmˌwʌn/
UK /ðə dʒoʊk ɪz ɑn ˈsʌmˌwʌn/

1.
o tiro saiu pela culatra, a piada é para alguém
used to say that someone who has tried to make another person look silly has in fact been made to look silly themselves
:
•
He thought he was being clever by tricking me, but the joke is on him now that I've exposed his lies.
Ele pensou que estava sendo esperto me enganando, mas o tiro saiu pela culatra agora que eu expus suas mentiras.
•
They tried to embarrass her with a prank, but when it backfired, the joke was on them.
Eles tentaram envergonhá-la com uma pegadinha, mas quando deu errado, o tiro saiu pela culatra.