Wiktionary
n. (context idiomatic English) money in the form of paper currency or coins, paid immediately at the time and place of a transaction.
Wikipedia
"Cash on the Barrelhead" is a song written by Charlie and Ira Louvin, known professionally as the Louvin Brothers, which was first recorded and released in 1956 as the B-side of "You're Running Wild". The single came at a high point in the Louvins' career, and the song's rollicking honky-tonk feel has led it to be frequently covered both in recordings and live performances.
Usage examples of "cash on the barrelhead".
Except for student tickets, student passes, whatever they call them, every seat in the house is paid for cash on the barrelhead the day of the game.
All these deals have to be put through cash on the barrelhead or in advance.
These were actually bales of American currency from Saudi Arabia, cash on the barrelhead, so to speak, for the McDonald's Hamburgers Division of RAMJAC.
James van Houghton wants only the best, and is more than willing to pay cash on the barrelhead.
Or you get ten thousand big ones, straight up and down, I peel them right off, cash on the barrelhead.