Using わかったもんじゃない


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?




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.




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.

