E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The very best wine. The word is Low Latin for upon the nail, meaning that the wine is so good the drinker leaves only enough in his glass to make a bead on his nail. The French say of first-class wine, It is fit to make a ruby on the nail (faire rubis sur longle), referring to the residue left which is only sufficient to make a single drop on the nail. Tom Nash says, After a man has drunk his glass, it is usual, in the North, to turn the bottom of the cup upside down, and let a drop fall upon the thumb-nail. If the drop rolls off, the drinker is obliged to fill and drink again. Bishop Hall alludes to the same custom: The Duke Tenterbelly exclaims Let never this goodly-formed goblet of wine go jovially through me; and then he set it to his mouth, stole it off every drop, save a little remainder, which he was by custom to set upon his thumb-nail and lick off.
Tis here! the supernaculum! twenty years
Of age, if tis a day.
Byron: Werner, i. 1.
Supernaculum. Entirely. To drink supernaculum is to leave no heel-taps; to drink so as to leave just enough not to roll off ones thumb-nail if poured upon it, but only to remain there as a wine-bead.
This is after the fashion of Switzerland. Clear off neat, supernaculum.Rabelais: Gargantua and Pantagruel, bk. i. 5.