The online dictionary Jisho says that this is a colloquial expression, but offers no examples of its use; and I can't find examples elsewhere online. I used it in a sentence intending to say, "We might stay at the club long enough to have breakfast with the band, but who knows…" 「多分朝ご飯はバンドと食べる、がわかったもんじゃない。。。」I was aiming for a light-hearted joke since everyone present knew that we don't have the stamina to stay up that late.

I was told that わかったもんじゃない did not work in my sentence, that the expression is not light-hearted. Could someone please offer some guidance regarding when to use/not use わかったもんじゃない?

  • 2
    One little thing - generally, が・けど etc. attach to the clause before them, rather than after, so the comma is in the wrong place there. I have heard them beginning sentences but very rarely
    – Angelos
    Commented Dec 6, 2022 at 18:27
  • Thank you! I didn't know that (obviously ).
    – NattoYum
    Commented Dec 7, 2022 at 0:17

1 Answer 1


わかったもんじゃない (or わかったものではない) is a set phrase that always implies something bad will happen. For example, 彼女は何をするかわかったもんじゃない implies she would almost certainly do something unexpected and bad, although it is hard to imagine what she would do exactly or how awful it would be. In your case, わかったもんじゃない did not work because you didn't know whether the result would be favorable or unfavorable. Simply, you could have said まだわからない instead.

  • Is there a simple explanation for what makes this phrase so ominous sounding? Without your answer I would have guessed that it was just a more emphatic version of わからない. Commented Dec 7, 2022 at 18:10
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    @user3856370 A set phrase doesn't always work literally. "You can talk" doesn't mean "You can say that", and うそ言え doesn't mean "Tell some lie!".
    – naruto
    Commented Dec 8, 2022 at 0:29
  • Sure. I guess my problem is that with many set phrases you can tell that they are set phrases because they're hard to make sense of in any other way. Whereas, this phrase seems to be perfectly grammatical and normal looking and I wouldn't even have guessed that it had a special meaning. Commented Dec 8, 2022 at 0:52
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    @user3856370 A perfectly normal-looking way of saying this is (事前に)分かることじゃない. This use of た and ものじゃない (negative of ものだ) make this look like an idiom to me.
    – naruto
    Commented Dec 8, 2022 at 1:57

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