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.
I have no idea where he went.
I can't even guess the answer to this problem.
Standard kanji spelling using つく.
Alternate kanji spelling using 付く; both are common.
From 見当 (estimate, guess) + が + つかない (negative potential of つく, here meaning 'to be formed' or 'to be established').