이럴 수가 (i-reol su-ga) – unbelievable/impossible
There are many ways of interpretting this phrase but it’s more or less used to explain how shocking and surprising something is.
It is more often than not used as a reaction phrase, so you would typically use this when you see, hear or experience something unbelievable/impossible.
A shorter version of this but a lot more colloquial word is 헐 or popularised by Reply 1988 – 웬열.
와 이럴 수가… 그 여자의 이야기를 들어서 진짜 대단한 여자다.
Wow, I can’t believe it… After hearing that woman’s story, she’s indeed an amazing lady.
아줌마가 한 번만 보는데 복권에 당첨했어! 이럴 수가.
Auntie just tried once and she struck the lottery! Unbelievable.