sharp as a tack
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Also, sharp as a razor. Mentally acute. For example, She's very witty—she's sharp as a tack. These similes are also used literally to mean “having a keen cutting edge” and have largely replaced the earlier sharp as a needle or thorn. The first dates from about 1900, the variant from the mid-1800s.
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Which of the following nouns has an irregular plural form?
Words nearby sharp as a tack
Sharm al-Sheikh, Sharon, sharon fruit, Sharonville, sharp, sharp as a tack, sharpbender, sharpbill, sharp-cut, Sharpe, sharp-eared
The American Heritage® Idioms Dictionary
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