Bunghole /ˈbəNGhōl / noun

Definition: Bunghole is a noun used in the English language to describe a hole or opening through which we fill a flask or cask. We can easily fill or empty a liquid-filled cask by using its primary opening or bunghole. People commonly carry casks filled with liquid for drinking, like water or alcohol, when traveling on long trips or camping outdoors.

Bunghole also has a secondary informal meaning used in vulgar slang that is descriptive of an anus. English speakers use the word as vulgar slang as another way to insult someone by calling them a butthole or jerk.

Etymology: The word bunghole showed up in the English language in the 15th century. It was used to define a hole in a cask with a stopper in 1520. 

Bunghole became a popular term used in vulgar slang around the early 1600s when people started using it as an insult akin to insulting or calling someone an asshole or anus.

In a Sentence

He poured out the whiskey through the bunghole of his flask.

That guy was sure acting like a bunghole!

Stop acting like a bunghole and mind your manners!

Synonyms 

Jerk, Arse, Filling Contraption 

Antonyms 

Sweetie, Dear