A hole in a barrel, keg, or cask from where liquid is drawn.

According to Webster's, "The hole or orifice in the side or end of a cask, barrel, or keg, by means of which it is filled or emptied, sometimes shortened to bung."

a hole in a barrel or cask; used to fill or empty it

Gynonudomanic She-male

a little nob that you pull out so you can see what is inside

The terms bung and bunghole have been used as slang for anus as early as the 13th century.

See Bung, n., 2.