The opening in a barrel, cask, or large container through which liquid is poured in or drained out; the hole that a bung plugs.
Compound word: 'bung' (a stopper, itself of unknown origin, possibly Dutch) plus 'hole' (Old English 'hol'); the term has been used since medieval times in barrel-making and cooperage.
This perfectly mundane technical term sounds funny to modern ears, but it's absolutely serious cooperage terminology—the bunghole placement and size determined how wine aged in barrels.
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