To have not the slightest idea; to be unable to guess or estimate something at all (same as 見当がつかない).
彼の居場所は見当もつかない。
I have no idea where he is.
この機械の故障原因は見当もつかないから、専門家に見てもらおう。
I have no clue what's causing this machine to fail, so let's have an expert look at it.
次の問題の答えは見当もつかない。
I have no idea what the answer to the next question is.
給料がどれくらい上がるか見当もつかない。
I have no idea how much the salary will increase.