(文頭に置けば)―There is no saying―(文尾に置けば)―for aught one knows―(独立句に用いれば)―Who knows?―God knows―(形容詞に用いれば)―unspeakable sorrow―inexpressible feeling―indescribable beauty―unutterable grief―ineffable joy―indefinable charm―exquisite pleasure
I can't say for certain.
There is no way of expressing it―It is unspeakable―unutterable.
何とも言えないな。
I can't say for sure.
I'm afraid I'm unable to say anything in regards to that.
I am dumfounded―I am struck dumb with astonishment―astonishment deprives me of my power of speech.
That's hard to say.
There is no saying what may happen.
I cannot say anything definite [for certain] at present.
He does not make many words about the matter
I'm not telling a lie, that's flat.