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sharp as a tack

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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RELATED WORDS

wise, shrewd, discriminating, sensitive, quick, sharp, astute, adroit, discerning, keen, canny, intelligent, crafty, insightful, vigorous, effective, deft, able, active, ready
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.