To take something quickly and quitely without anyone noticing. Offen used in the online game WoW when a party member takes an item without the party's consent.
Tim: Dude you gotta see my new phone it's so boss
*tim reaches in his pocket for his phone*
Tim: What the hell, who ninjad my damn phone?
Mike: oh dude I thought you knew I was looking at it like 5 minutes ago.
Tim: How the??