out of one's depth
超出了某人知识[理解力]的范围； 束手无策；力所不能及；力所不及

【例句】
If you get him talking on art, he is soon out of his depth.
如果你和他谈论艺术, 他很快就不知该如何作答了。

Derek is out of his depth with the new computer.
德列克不会操作这部新电脑。