* A humorous or invented term: Someone might use it playfully to describe a simple, perhaps poorly made, musical instrument or the sound it makes.
* A slang term: There might be a specific community or subculture where this term is used with a particular meaning.
* A misheard phrase: You may have misheard a different term or phrase.
Could you provide more context?
To help understand what "plunk twanger" means, tell me:
* Where did you hear this term? (In a song, a movie, a book, a conversation, etc.)
* What was the context? (What was being discussed or described?)
With more information, I might be able to figure out the intended meaning.