Nerf is a slang term for when a module creator or another developer changes something to weaken it. For example, changing the spell Bull's strength to give only 2 ability points instead of 1d4+1 might be referred to as "nerfing" it -- the opposite of buff.

The term "nerf" comes from the name of a soft squishy foam ball that is marketed to parents to purchase for their adolescent children as safe indoor toys that could not possibly hurt anyone no matter how hard they were thrown. Nerf balls today are marketed towards frustrated suits in office cubicles as a form of stress relief.