A Stab in the Dark
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If you stab in the dark, you attempt to do something but you cannot actually see the target you are focusing on. If you are trying to answer a question, to take a stab in the dark is just to guess.
To stab is to attack somebody with a knife. Literally, a stab in the dark is trying to attack someone, whom you cannot see, with a knife. These expression comes from legends in which heroes had to fight with invisible enemies and the only way to defeat them was by stabbing them in the dark.
Example:• -Hi, John. How was your last History exam?
• -It was horrible. I didn´t study anything at all. I didn´t know we had an exam. It all came like a nightmarish surprise.
• -Then, you want to tell me that you didn´t answer any questions.
• -Of course, I did. I answer all the exam but I was just taking educated guesses.
• -So you mean that you were just stabbing in the dark.
•-Well , yes. We could say so.