Proc (I'm not even sure if LotRO has weapons that proc - I'm still new to the game). No one's REALLY sure the acronym stands for exactly, but it's a common mmo word that is generally accepted to be short for 'Procedure' although some believe that it stands for either 'Process', 'Programmed Random Occurance, or possibly Potential Rate Of Cast (this one is my favorite). It can also be thought of as an abbreviation of 'Proclivity', meaning a tendency or inclination.

Whatever the case may be, this abbreviation refers to when a weapon or item activates with a "Chance on Use" effect (an ability or a spell).

So if you have a magic sword that has a chance to cast an extra 25 fire damage while fighting, every once in a while the sword would randomly "proc" an extra 25 damage.