This refers to the foam toys made by Mattell. When used in game it means to render a class or ability weaker. Usually done by the developers through patches.

a change to the rules of the game that is generally considered to have a weakening or negative effect upon the affected object(s). This term elates to how Nerf toys for children use safe use soft foam. Developers nerf aspects of the game in order to maintain game balance.

The action of the Devs weakening something in the game, usually a skill or item mod.

When a skill or a class gets some of its benefits lessened or removed, in order to balance the game, that skill or class gets nerfed.

For the administration to change something in such a way that it is weaker or less effective.

term used when something in game have it power reduced (read tutorial)

The act of reducing the effectiveness of a class or skill when it becomes apparent people are succeeding at it. This process happens to every class and every skill at some point. It is believed that by the year 2003 all the classes will have only hand to hand combat and no spells except Identify Item, and will be able to use Sense Heading only, at which point the game will be declared perfectly balanced. After the following patch, in the name of game balance, Identify Item will be removed from all spellcasters.

A verb, meaning to weaken something considerably. Used in D2 in reference to Blizzard 'toning down' certain skills in patches, most recently corpse explosion and whirlwind.

A nerf is a change to the rules of a computer game that weakens a certain object or ability.

Nerf (an acronym for Non-Expanding Recreational Foam) is a product originally introduced as an indoor-safe ball by Parker Brothers in 1969. Created by Reynolds Guyer, who carved the prototype out of foam, Nerf was an instant success. Sales of the balls mounted to four million in the year of their inception.

Light contact with another car, usually from behind. Many times the following driver will "nerf" the car ahead to put pressure on an opponent.