Half stout and half porter. The term arises from the practice at breweries of allowing the coopers a daily portion of stout and porter. As they do not like to drink porter after stout, they mix the two together.
A coop for wine bottles. The bottles lie in a slanting position in the coop, and may be transported in it from place to place. We find allusions to “six-bottle coopers” not unfrequently, i.e. coops or cases containing six bottles. Compare “hen-coops,” “cooped up,” etc. (Latin, cupa, a cask; our “cup.”)
“(Enter waiter with a cooper of wine.)”
Waiter: “Six bottles of wine for Corporal Toddy.” O'Keeffe: Rogues All, iii. 4.
Do you want a cooper? This question is asked of those who have an order to visit the wine cellars of the London Docks. The “cooper” bores the casks and gives the visitor different wines to taste.