Stunning

A Stunning (pre-Update 5, Weighted) weapon adds to the DC of your Stunning Blow and Stunning Fist check. Any Stunning weapon also has a 3% chance to make a fortitude save check to stun an opponent on any attack. Fortitude DC on stun check depends on + written on it.
 * of Stunning +2 - DC 25
 * of Stunning +4 - DC 27
 * of Stunning +6 - DC 29
 * of Stunning +8 - DC 31
 * of Stunning +10 - DC 33

Example: +1 Warhammer of Stunning +10 would give +10 to the DC of any Stunning Blow attempts made with that weapon. This weapon would also have a 3% chance on-hit to make DC 33 fortitude check to stun the monster.

Note: While Two-Weapon Fighting, the increased DC of your stunning blow attempts applies to both weapons, however the 3% chance to stun only applies to the weapon with the weighted property. (unconfirmed)

The Stunning property is only available on bludgeoning weapons and handwraps.