でなくてなんだろう

JLPT Level: N1

Meanings

If X is not Y, then what is it?

What else could it be if it's not X?

So if not X, then what is it?

Usage

Noun + でなくてなんだろう

Na-adjective + でなくてなんだろう

Verb dictionary form + でなくてなんだろう? (less common)

Explanation

This expression is used when you’re speculating about what something is or what its role could be, given that it is not the expected or assumed thing. It conveys surprise or curiosity and invites a consideration of alternatives. It often appears after stating that something is not something else, followed by a question about what it might be instead. It can carry a gentle, reflective, or slightly surprised tone rather than strong certainty.

この模様は鳥の尾ではなくてなんだろう。

このもようはとりのおではなくてなんだろう。

I wonder what this pattern is if it isn’t a bird’s tail.

彼の言い方は友達でなくてなんだろう。

かれのいいかたはともだちでなくてなんだろう。

I wonder what his way of speaking is if it isn’t friendly.