Dictionary.net

Good liquor, of which not enough is left to wet one's nail.

A kind of mock Latin term intended to mean upon the nail, used formerly by topers. Nares. "To drink supernaculum was an ancient custom not only in England, but also in several other parts of Europe, of emptying the cup or glass, and then pouring the drop or two that remained at the bottom upon the person's nail that drank it, to show that he was no flincher."-Brand: good liquor, such as one will drink till not enough is left to wet one's nail.