nail [OE] When the word nail emerged in the Old English period it already had its main modern meanings of ‘small metal spike’ and ‘fingernail’. To nail a lie is to expose a falsehood, an idiom known from the early nineteenth century. The reference is most likely to shopkeepers nailing forged coins to their shop counter to expose them and put them out of circulation. If money is paid on the nail it is paid without delay, immediately. There are similar expressions in many other languages. The phrase may come from the Satires of the Roman poet Horace, who used ad ungulum, ‘on the [finger]nail’, to mean ‘to perfection’ or ‘to the utmost’. This referred either to Roman sculptors making the finishing touches to their work with a fingernail, or to carpenters using a fingernail to test the accuracy of a joint. An American equivalent was on the barrelhead, an upturned barrel being a simple shop counter.