From Distillers Wiki
In cooperage, a wooden stopper used to seal the cask, keg or barrel. In glassware, usually a rubber stopper used to seal a demijohn, carboy or jug. Bungs may be either solid or drilled with a central hole to accept a fermentation lock (airlock). Some bungs have two holes drilled to accept two airlocks, or one airlock and a blow-off tube.