A salve said to cure wounds by sympathy. The salve is not applied to the wound, but to the instrument which gave the wound. The direction “Bind the wound and grease the nail” is still common when a wound has been given by a rusty nail. Sir Kenelm Digby says the salve is sympathetic, and quotes several instances to prove that “as the sword is treated the wound inflicted by it feels. Thus, if the instrument is kept wet, the wound will feel cool; if held to the fire, it will feel hot;” etc.
But she has taen the broken lance, And washed it from the clotted gore, And salved the splinter o'er and o'er.
If grease must be used to satisfy the ignorant, it can do no harm on the rusty nail, but would certainly be harmful on the wound itself.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894