Sharpness is a temperament term used mainly in working German Shepherd circles but can be used to refer to the temperament of any dog.  There is no common definition for the term, but in general it refers to a dog that reacts quickly with serious "fight drive" aggression toward people with very little provocation.

It can be a confusing term since many dogs are reactive and act aggressively toward people, especially dogs that have an all around more fearful temperament to everything.

Usually it used with dogs that have a more functional degree of confidence in the world and the switch to an aggressive state seems more extreme, confident, and unexpected.  Without formal training a "sharp" dog can unexpectedly bite someone if a handler lets their guard down.  With formal training it can make protection style training easy to put a command to aggression.

Here is an example of a "sharp" dog: