The phrase “cash on the barrelhead”: Immediate payment; spot cash; payment on the nail.
There is no doubt in my mind that the original scene here was the makeshift bar of the American frontier a century or more ago.
The barrel itself, under improvised conditions, served as both container and counter, or, when empty, as counter only.
But the wise bartender trusted none of his customers and extended no credit. Nor did he dare turn to serve another lest the first vamoose, and he unable to give chase.
The expression dates back many years, but, regrettably, none but recent uses have been found.
Its modern equivalent is the weak “cash on the counter.”