(comparative more on the nail, superlative most on the nail)
Allegedly from the tradition of striking bargains by placing cash on the nails in Bristol. 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’."