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

1.
şaka birinin başına patlamak, şaka birine dönmek
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.
Beni kandırarak zeki olduğunu sanıyordu ama yalanlarını ortaya çıkardığıma göre şaka onun başına patladı.
•
They tried to embarrass her with a prank, but when it backfired, the joke was on them.
Onu bir şakayla utandırmaya çalıştılar ama şaka ters tepince şaka onların başına patladı.