The phrase "sharp as a tack" is an idiomatic expression used to describe someone who is very alert, mentally quick and highly intelligent. It means that the person in question has a clear mind, is fully focused and able to think quickly and make decisions with ease. The origin of this phrase is unclear but it may have come from the fact that tacks are small sharp objects used for fastening things together, so being "sharp as a tack" would mean having a very keen and alert mind.