Definitions
1.verbTo bump lightly, whether accidentally or purposefully."A racer will often nerf another as a psychological tactic."
2.nounThe deterioration, weakening or worsening of a character, a weapon, a spell, etc.
3.verbTo cripple or weaken an element of a video game during its development (such as a character, a weapon, a spell, etc.)."The lightning spell was originally pretty powerful, but in the sequel they nerfed it so it became completely useless."
4.verbTo arbitrarily limit or reduce the capability of.