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 l'ongle), 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 supernaculuin! twenty years Of age, if't is a day.
“This is after the fashion of Switzerland. Clear Off neat,
Their jests were supernaculum, I snatched the rubies from each thumb, And in this crystal have them here. Perhaps you'll like it more than beer.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894