This ability is learned with 10 skill points allocated into Acquisitiveness skill. The actual success rate is dependent on both the user's Deftness and the drop rate of the item, calculated as Steal Rate = (Drop Rate * 2)(1 + (Deftness – 51) / 474)
If the character is holding the Honor Among Thieves re-vocation medal, their chance to swipe an item will flat out double. This will not effect other calculations that use the deftness stat, however.
Half-inch is a Cockney English rhyming slang that means 'to steal (pinch)'.