Weaken is a level 11 spell that requires a body rune. It reduces the target's damage dealt by 5% for 1 minute, making it valuable in player against player. Still, it is not used much in battles because of the turn you lose.
Casting Weaken while wearing full metal armour (including an elemental staff with no more than +10 magic points) guarantees a "splash," or a missed hit. This provides very cheap magic experience, as players can instantly recast the spell on the same target. Additionally, casting Weaken grants players no experience in the Constitution skill and, thus, is useful to pures.
The usual method is to buy the Body runes needed for the cast, as well as an appropriate amount of one of the other two runes, and using the respective staff of the third rune. An expensive alternative is to buy a Mud Staff, which provides an unlimited supply of both Water and Earth Runes. Note that full bronze does not supply a high enough decrease in magic bonus while wielding a staff, so full iron is recommended.
When doing this method, it is helpful to position a monster so that it is just on the edge of your screen, so that there is minimal distance between it and your spellbook.
Monsters such as Elemental wizards, Dark wizards and Catablepons are mystical in nature, and are capable of casting Weaken upon players. This effect does not wear off even if you are logged off, however it will wear off within minutes at most if you stay logged in.