war chalking
n. Using chalk to place a special symbol on a sidewalk or other surface that indicates a nearby wireless network, especially one that offers Internet access.
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Mr. Jones said by e-mail that he "had a light-bulb moment" after seeing architecture students "chalking up the pavement" on his way to lunch. At lunch, Mr. Jones and a friend, who had recently been discussing hobo signs with another friend, came up with the notion, which Mr. Jones dubbed "warchalking."

Warchalking derives from "war driving," a pastime of discovering wireless networks using widely available data-sniffing software that identifies open and restricted wireless nodes in homes and businesses. (War driving, in turn, comes from war dialing, modem-era hacker attempts to dial number after number to find modems.)
—Gless Fleishman, “Road Signs for Vagabond Computer Users,” The New York Times, July 11, 2002
2002 (earliest)
Wi-Fi, the standard for broadband wireless networks, has grown rapidly over the past year as companies and individuals implement "hot spots" that offer wireless Internet connectivity within a short range, usually about 100 feet.

The networks are often available in public places, including Starbucks shops and airports. The trouble is, it's hard to know where many such hot spots are available, if they're free and if they're unsecured so anyone can use them. Matt Jones, a London resident who swears he's not a geek, recently planted the seed for solving that problem and the idea has quickly spread around the Internet. His plan is based on a piece of hobo culture where wanderers would mark symbols on homes or buildings indicating where other hobos might work for food or find a warm place to sleep.

Jones proposed on a Web log that Wi-Fi users agree on a set of symbols that they can chalk on sidewalks or buildings to help other users find out where they can get on to the Internet. . . . Jones calls it "warchalking," a play on the hacker term "wardialing," which refers to a hacker practice of setting computers to continually dial phone numbers until they reach one that lets them break into a network via the modem.
—Nancy Gohring, “Chalk up a novel plan for Wi-Fi users,” The Seattle Times, July 01, 2002