Source : Free On-Line Dictionary of Computing

warchalk
     
         A system of runes and annotations chalked on
        walls or other surfaces to indicate to interested parties the
        presence of a {wireless} network {node} in the vicinity.
     
        Warchalking was inspired by "hobo language" - the signs used
        by American itinerants during the Depression years to indicate
        where they might find a meal.
     
        {Home (http://www.blackbeltjones.com/warchalking/)}.
     
        (2002-06-26)