A weapon salve is an attempt to heal a wound caused by a bladed weapon with an herbal preparation applied not to the wound itself but to the blade of the weapon that caused it. You can see how this won't work.
This belief in action at a distance was common to practitioners of Paracelsian medicine (Paracelsus, incidentally, coined the concept of "the dose makes the poison"). Typically the salve would include blood from the wound caused by the weapon.
Paracelsus's own recipe for such a salve:
“”Take of moss growing on the head of a thief who has been hanged and left in the air; of real mummy; of human blood, still warm – of each one ounce; of human suet, two ounces; of linseed oil, turpentine, and Armenian bole – of each two drachms. Mix all well in a mortar, and keep the salve in an oblong, narrow urn.
The practice was a form of sympathetic magic. The weapon and the wound are joined magically, since one caused the other. The ingredients for the salve itself all have magical properties associated with healing, with the alignment of the stars, or with other magical processes. The human blood is a form of life magic, common in any sacrificial ritual.
- J.N. Hays. 2009. 'The Burdens of Desire: Epidemics and Human Response in Western History. Rutgers University Press. P. 97. ISBN 9780813546131.