Allegedly from the tradition of striking bargains by placing cash on the nails
. The Oxford English Dictionary however cites a Anglo-Norman phrase from c. 1360, "payer sur le ungle" to pay on the (finger)nail
, meaning "to pay immediately and in full", and quotes parallel usages from 17th century French, Dutch and German sources. It adds that "N.E.D. (1906) notes that: ‘the explanations associating it with certain pillars at the Exchange of Limerick or Bristol are too late to be of any authority in deciding the question’."