sharp

锋利之意

tack

大头针是也

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成语也罢，idioms也罢，都是历经时间的检验所沉淀下来的最形象最有说服力的词汇。

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如大头针一般锋利，就是很快解决问题，就是说一个人很聪明，很灵性等。

(as) sharp as a tack

If someone is as sharp as a tack, they are very clever indeed.

He may be old in years, but he's still as sharp as a tack and knows what he's talking about.