넌 대체

who/what on earth are you?

→ 너+는
phrase Rare #24,572
who/what on earth are you? 넌 is a casual contraction of 너는 (“you”); 대체 adds emphasis, often conveying surprise or frustration.
KO: 넌 대체 왜 여기 있어?
EN: Why on earth are you here?

넌 대체 누구 편 이야?

Whose side are you on anyway?

— 스틸 라이프 (2006)

넌 대체 누구 냐?

Who the hell are you?

— 너자 (2019)