A large bag of alcohol (typically wine), generally very cheap, rancid on the palate and housed (temporarily) in a box. Typically a lower socio-economic choice of beverage.
Often followed by violence upon or by the person consuming said flagon.
NB: Flagon can be inflated post-consumption to provide a pillow. Nice.
"Hey Morton you dardy mutt, come aroun' to our place and bring a flagon of woobla. Der's a party happening tonight"