Ok, first a little history for the non-techs. Back when modems were the primary mode of communication between even large computers, Bad Guys would set up their computer to dial hundreds of numbers, and see if there was a computer on the other end. Then they’d have a short list of computer numbers, and they’d try to break into them. This was called "War Dialing".
In the last 5 years or so, people have started driving through neighborhoods with wifi detectors, looking for open wireless networks. This has been dubbed "War Driving".
Very often, once an open network is found, people will use chalk to make a mark nearby to indicate that there’s an open network. This then became "War Chalking".
Near my house are two restaurants across the street from each other (Jukebox Joe’s and The Cheshire), and each serves good ice cream. Someone used paint to indicate that in the street between them, and I’ve dubbed it "War Painting".