I'm not a teacher
Hi jesson cui,
pay on the nail = cash on the barrelhead = immediate payment, as in
They won't extend credit; it's cash on the barrelhead or no sale
The Nail = In the centre of The Limerick Stock 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.
Student or Learner