anybody's (或 anyone's) guess

very difficult or impossible to determine

很難確定；誰都拿不準

how well the system will work is anybody's guess.

這一系統(tǒng)的運行效果如何，誰也拿不準。

keep someone guessing

(informal)leave someone uncertain or in doubt as to one's intentions or plans

(非正式)使（某人）捉摸不透自己的意圖（或計劃）