Deflect is currently a little understood and little used mechanic, the only place it's been seen in game is when a player is being attacked by an NPC that is using an ability that would give players the drunk effect. A deflect negates all the effects of these attacks if it occurs. It is not know what the chance is to deflect nor if it requires any type of weapon or offhand equipped.
You can also see it by typing in game:
For auto-attacks 'deflect' :
/script message( VSDEFLECTOTHEROTHER ) /script message( VSDEFLECTOTHERSELF ) /script message( VSDEFLECTSELFOTHER )
For abilities (or maybe spells ?) 'deflect' :
/script message( SPELLDEFLECTEDOTHEROTHER ) /script message( SPELLDEFLECTEDOTHERSELF ) /script message( SPELLDEFLECTEDSELFOTHER ) /script message( SPELLDEFLECTEDSELFSELF )
Template:Damage Avoidance