Textbook vs Real
Textbook그것은 말이 되지 않습니다geu-geos-eun mal-i doe-ji an-seum-ni-da
Real헐, 말도 안 돼heol mal-do an dwae
Why Koreans say it this way
K-drama disbelief combo. 헐 (OMG, pure exclamation — no textbook equivalent) + 말도 안 돼 (lit. "words don't become" = "that doesn't make sense"). The textbook version sounds like a legal objection.
From: K-Drama Common