One of the few new human members of the modern Ankh-Morpork City Watch.
As he is quick to tell anyone who does or doesn't ask, ping is a dialect word meaning "watermeadow"; that's also the what fellow watchmen call him. He's the type who is always prepared to be questioned, slightly apprehensive, and therefore prone to a lot of bad luck and minor mistakes. In Night Watch he merely asks about the lilacs he sees the watchmen Fred Colon, Nobby Nobbs and Reg Shoe wear on the twenty-fifth of May every year, which causes the normally friendly Sergeant to become rather angry. In The Fifth Elephant, he is the officer who discovers the theft of the replica Scone but misses the thief.
There are two possible references to the name - Ping is an application within the tcp/ip network protocol used to confirm connectivity between two computers. One computer send a short signal (or packet) to another, which then returns the packet to confirm its existance. The name of this application derives from the sound made by Active SONAR when an echo is returned.
However it could also be a simple reference to a sterotypical oriental style name, such as Hong and Tang (both also used by TP)... A Chinese word pronounced "ping" can be translated to a "plain, or meadow".
According to this boing-boing post, Sir Terry actually named Constable Ping after the poster, who told him that his name was a Middle-English word for a watermeadow.