it means basically 'cash and carry'... you pay someone cash on the spot (placing the cash atop a barrel that's for sale) and they in turn release the goods to you right then and there - and you carry it (the barrel) away!
I see that you're selling your car on line. Can I give you cash on the barrel head and I drive away with the title?
also "cash on the barrelhead". Means that you give me cash, and I'll give you the product at that time. No credit no delivery.
Buddy: You ever think about selling that old juicer?
Barbara: I'll give it to you for $20 - cash on the barrel.
Buddy: OK, here is the money.
Barbara: Thank you, here is the juicer.
Buddy: It has been a pleasure doing business with you.
Barbara: The feeling is mutual.
Fogey/fogy /fougi/ sl. (early 18C+, orig. Scot) old-fashioned, stuck-in-the mud.
Person with old fashioned ideas which he is unwilling to change: Come to the disco and stop being such an old fogey!
You think me an old fogeyand an old tory, his thoughtful voice said. I saw three generations since O’Connel’s time. I remember the famine. Do you know that the orange lodges agitated for repeal of the union twenty years before O’Connel did or before the prelates of your communion denounced him as a demagogue? You fenians forget some things. (James Joyce, Ulysses. Penguin Books,1992. p. 38)