Cripple

Fallout & Fallout 2
To cripple an enemy, aim for the arms and legs. If you have a crippled leg you can't run and you use more Action Points. If you have a crippled arm you can't wield two handed weapons. If the arm crippled is the arm wielding a gun, the accuracy of the weapon will decrease significantly. If both arms are crippled, you can only use unarmed attacks.

In fallout 2, crippling of the eyes resulted in blindness

Fallout 3
Although body part damage can be achieved through manual aiming, VATS allows more effective targeting of particular body parts. The effects of crippling specific body parts is listed below.
 * Crippling the head will reduce overall accuracy and perception of the target. If inflicted upon the player, the screen will intermittently get blurry and a ring will be heard through the speakers. A status condition of Concussion is displayed.
 * Crippling the torso will result in the target flinching far more often from shots than normal.
 * Crippling the arm of an enemy will result in any weapon reliant on that arm for holding to be dropped, and will also drastically reduce accuracy. The weapon itself is not damaged, and the enemy will likely pick the weapon up again anyway. If inflicted upon the player, overall accuracy is reduced, but the currently equipped weapon is not dropped.
 * Crippling a leg will result in impaired mobility. On creatures with a leaping attack (like Deathclaws), they will no longer be able to do so.
 * Crippling the held weapon will ruin it and make it unusable, and will fly out of the target's hand. If it is an explosive ordinance, like a frag grenade, it will explode, dealing damage to the target holding it, and cause splash damage to anything around it. Sometimes, destroyed grenades can still be reclaimed, but they will be relatively useless.
 * Some enemies have alternate targets: Robots, ants, and other such creatures have special targetable parts that, when crippled, will force them into a frenzied state, where they no longer discern between friend and foe and will attack anything around them..
 * Certain weapons (such as the Railway Rifle and Dart Gun) are more adept at crippling body parts.

Crippling any body part will result in the target momentarily puasing to clutch at the crippled body part. This can be very useful during combat, as it may give you a few precious seconds in which to finish off your target. When using explosive weapons, such as grenades, you cannot target specific body parts while in V.A.T.S. You simply target the enemy as a whole.

Healing a crippled body part
In combat, the player may also sustain crippling damage. In order to repair this damage, you must:
 * Pay a doctor to heal you
 * Sleep in a bed
 * Use a stimpack