For those who don't know, warchalking is the practice of using simple signs, usually chalked onto a sidewalk or building wall, alerting people to an open WiFi connection. Anyone with a wireless connection could use the open connection to connect to the Internet. Some of these connections are purposely open, such as when Starbucks or other coffee shops offer wireless Internet access, others are open because a user did not bother to configure their home network properly to block unauthorized usage.
A Zone-H article from 2002 goes into the history of the word:
The "war" part of the term goes back to the 1983 US movie War Games, in which a teenager accidentally taps his home computer into the Pentagon. Before warchalking there was "wardialling", when hackers would ring phone numbers at random to see which ones answered with a data, rather than a dial, tone, and "wardriving", travelling around in a car until the laptop picks up a network.Except for getting the details about WarGames wrong -- the computer David hacks into, "Joshua," is located at NORAD, not the Pentagon -- that seems to sum up the origin of the word. It also answers the question about whether it should be warchalking (one word) or war chalking (two words).
The idea of the chalk marks comes from "hobo signs", an old tradition among travellers and the homeless in America. They would scrawl a mark on a wall to show other hobos where they might find food or shelter. The circles, semi-circles and letters used in warchalking give computer-users the information they need to tweak their wireless network setting to access other people's connections.
There are a couple of pages that give examples of hobo signs [ Signs I | Signs II | Signs III ], some of which are pretty ambiguous, others are easier to read. Digging around some more, I also find a sport, hashing, that also uses cryptic symbols as part of a complicated game of chase. According to the Charlotte Hash House Harriers (CH3):
"Hashing is a mixture of athleticism, sociability, hedonism and hard work; most of all its a refreshing break from the grind and a chance to drink beer with great friends. A Hash Trail is an exhilaratingly fun combination of running, orienteering, and partying, where bands of "Harriers" and "Hairiettes" chase hares or follow marks of flour on 5-to-10 kilometer (3-6 miles) long trails through town, the woods, sewers, even the airport, all in search of exercise, camaraderie, and the ever elusive "Beer Stop."The "Hash Primer" on Half-Mind describes it as "the running club with a drinking problem" ... hurm, maybe that would make me enjoy running?