very
smart or intelligent; quick to realize, understand or learn.
Sharp means
having a
keen cutting or piercing
edge. A
tack is a
short,
sharp-pointed
nail having a
broad and
flat head, which is used for nailing down carpets, pinning something that is
thin or light to something heavier or stronger (such as a poster to a
wall), etc. Sharper than a
tack, used figuratively, refers to a person's intellectual keenness of wit and emphasizes someone's
intelligence or wit by comparing them to the sharpness of a
tack. It dates from the
late nineteenth or early twentieth century.
(colloquial) very quick to learn or realize things.