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

1.
누가 웃음거리가 되다, 누가 망신을 당하다
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.
그는 나를 속여서 똑똑하다고 생각했지만, 내가 그의 거짓말을 폭로했으니 이제 그가 웃음거리가 되었다.
•
They tried to embarrass her with a prank, but when it backfired, the joke was on them.
그들은 장난으로 그녀를 당황하게 하려 했지만, 역효과가 나자 그들이 웃음거리가 되었다.