To Pay on the Nail
This expression is still used to denote prompt payment. It arose from a practice in medieval markets, which were critical to the general buying and selling of wares, produce and livestock. Instant justice was dealt to those who reneged on agreements or cheated their customers and eventually it was decided that accounts be settled at counters (which were short pillars known as nails) in the open market place and in front of witnesses. Payments were placed on these counters for people to see that all the money and change was correct and had been settled and that bargains had been kept. Hence the custom of paying on the nail.
Words by Adam Jacot de Boinod, author of The Meaning of Tingo and Other Extraordinary Words from around the World, published by Penguin Books and creator of the iPhone App Tingo, a quiz on Interesting Words