1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): *sharp as a tack
Go to *sharp as a razor
McGrawhill's American Idioms And Phrasal Verbs