bung
Synonym discussion
Sense: noun/1
Bung, stopper, plug, cork, and cap overlap, but they are not used in exactly the same places. A bung is most natural for a barrel, cask, or large drum, especially in brewing or storage. Stopper is the broad everyday word for something pushed into an opening to keep liquid or air in. Plug is also broad, but it often stresses a tight fit and can name many other things, including electrical parts, so it feels less specific. Cork is right when the piece is actually made of cork, especially in wine bottles. Cap differs because it fits over the top from outside. You remove a bung from a cider barrel, but you put a cap on a water bottle.
Etymology
The older stopper sense goes back to late Middle English and came into English from Middle Dutch words for a plug or stopper. That history matches the word's long use around barrels, casks, and other storage containers. The verb developed later in British English, first for blocking an opening and then for putting something somewhere quickly and carelessly. The bribery sense is much newer and figurative, using bung for money slipped to someone so they will bend the rules.
First known use
15th century
15th century, in the meaning defined at sense 1 Verb