What's a thief to do if he losses his "on hand"?

[Jdvn1]

There's nothing in the rule to stop you from randomly choosing which hand would be your "on hand" and which would be your "off hand." Ambidexterity isn't a feat in 3.5 because anyone can be ambidextrous.

 

[Sejs]

Is the item something that's always on you? Like a manacle you wear that has your arm end at the wrist, or was it something that they put on you once and activated, resulting in your newfound handlessness?

 

[Primitive Screwhead]

I agree with enforcing the 'off-hand' penalties for fighting and a similar penalty on manipulative tasks like picking locks.

In my campaigns I have always had the rule of designating primary hand/off hand. I do this partially for RP reasons but also have some mechanics for things like spiral staircases that can impact on combat.
Designation is by a rule from back in 1e days, I think from a Dragon Mag but its really fuzzy that long ago. The rule is: Roll 1D20 and 1D8. The D20 represents your right hand, the D8 represents your left. Which ever is higher is your dominant hand. If they are equal, you are naturally ambidexterous.

Side note.. I am all for roguish justice of cutting of a hand..and this act should end the rogues career in the delicate arts. I am not sure about an item that voids regeneration or healing... altho by the RAW if you have a magic item making your hand 'not there', doesn't your hand come back in an Anti-magic field?
That would be weird!

 

[Tiberius]

altho by the RAW if you have a magic item making your hand 'not there', doesn't your hand come back in an Anti-magic field?
That would be weird!

Not if the device's effect was Duration: Instantaneous. Otherwise yes, wandering around inside an AMF would be a great idea for the rogue.