A bunghole is the hole in a barrel through which it is filled. Stop the bunghole, tap the bunghole, seal the bunghole.
The plug that closes it is the bung, and it is knocked in and out with a mallet. It is in the side of the barrel rather than the end, so the cask can be filled while lying down. Hamlet has a passage about stopping one, which is where most readers meet the word.
1nounbungholeThe hole in a barrel through which it is filled.
“He pulled the plug from the bunghole and filled the barrel with wine.”
bunghole is an opening.
bung-hole comes from bung + hole.
Etymology from Wiktionary, CC BY-SA