E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Down on the nail, Pay down on the nail. In ready money. In Latin: Super unguem; in French: Sur longle; as, Boire la goutte sur longle (see SUPERNACULUM), Payer rubis sur longle, where rubis means red wine. The Latin ungulus (from unguis) means a shot or reckoning, hence ungulum dare, to pay ones reckoning.
Quo quibus prisis, et cariagiis pleana flat solucio super unguem.An Indenture dated July 15th, 1326 (Scots Act)
OKeefe says: In the centre of Limerick Exchange is a pillar with a circular plate of copper about three feet in diameter, called The Nail, on which the earnest of all stock-exchange bargains has to be paid. (Recollections.)
A similar custom prevailed at Bristol, where were four pillars, called nails, in front of the Exchange for a similar purpose. In Liverpool Exchange there is a plate of copper called The Nail, on which bargains are settled.
Hung on the nail. Up the spout, put in pawn. The custom referred to is that of hanging each pawn on a nail, with a number attached, and giving the customer a duplicate thereof. Very similar to the custom of guarding hats, cloaks, walking-sticks, and umbrellas, in public exhibitions and assemblies.
To hit the nail on the head. To come to a right conclusion. In Latin, Rem tenes. The Germans have the exact phrase, Den Nagel auf den kopf treffen.