A flexible plastic container, usually housed inside a rigid cardboard box, designed to hold 4.5 Imperial gallons of beer. It functions as an inexpensive, disposable alternative to traditional metal or wooden casks. The collapsible inner bag prevents oxygen from reaching the beer as it is drawn off, extending the shelf life of real ale for home consumption or small events.
Heard on the job
Fill a polypin with the bitter so the customer can serve it on gravity at their weekend party.