Otherwise known as a 40oz for the newer generation, in the older generation of our parents, it is short-hand for a Giant Imperial Quart; a quart being 32oz, an imperial quart being 38.4 oz, and a giant imperial quart being 40oz.
Father: "Back in my day alls we did to relax was drink a couple of G.I.Q.'s"
Son: "Drink a couple of what?"
Father: "It's what you call a 40oz, it means Giant Imperial Quart"
Son: "Oh why didn't you say so? Me and my friends drink those all the time"
Father: "You WHAT?!?"