BODYSKIN is a modification for Quake made by David Wiedenmann. This is intended to be used as a minor add-on patch for MultiSkin and was created when Dennis Noordsij was on vacation as an add-on for 1.1. As MultiSkin was never updated, however, this functionality was never added to the main mod.
In regular MultiSkin, the player's corpse will still be the vanilla Ranger skin, regardless of the skin chosen. This changes the behavior so that dead bodies share the same skin as the player, allowing for varied textures for player corpses.