expression
have no idea; can't even guess
Used when one cannot form even a rough estimate or guess about something. Often implies complete lack of information or clue.
彼がどこに行ったのか、見当がつかない。
I have no idea where he went.
この問題の答えはまったく見当がつかない。
I can't even guess the answer to this problem.
From 見当 (estimate, guess) + が + つかない (negative potential of つく, here meaning 'to be formed' or 'to be established').