To send a signal or make brief contact.
Recently from computer networking, where one computer (usually a client) will send a quick, pointless query to another computer (usually a server) in order to find out if it is still online and see how long the response takes.
Originally from the naval warfare practice of "active sonar" where one ship (usually a surface ship) will send out a given sound or "ping" in order to measure the direction and delay in the "echo" of that sound off of another body (usually a submarine.
> Can we go to the movie tomorrow?
> I don't know... Ping me around eight and we'll see how things are going.