MP lost for the target

is a weapon effect that has a chance of reducing the MP from the attacked target.
Note: To be sure if the weapon really reduces MP from the target and not from your character, check the colour of the -MP, if it's red then it will reduce your MP, if it's black/green then it will reduce the target's MP.
Weapons that drain MP from the target are listed in :Category:MP Reduction weapons, or in this list:
<DynamicPageList>
category=MP Reduction weapons
allowcachedresults=true
</DynamicPageList>
Equipment that actually reduces the MP of the wielder are listed in :Category:MP penalty equipment, or in this list:
<DynamicPageList>
category=MP penalty equipment
allowcachedresults=true
</DynamicPageList>
Category:Game information