War-Chalking

War-Driving is the act of driving around in a car, searching for wireless access points. Once these access points have been found, it's common to mark them with some special chalk marks so that other hackers can find them.

Oftentimes a hacker will simply draw some symbols with chalk (hence the name War Chalking), identifying the access point to any other hacker that is in the area.

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Jargon dictionary definition
War Driving, War Flying and War Chalking With the advent of extremely inexpensive wireless access points, a new hacking technique called war driving has appeared.