What is fartknocker?

Fartknocker is a slang term, generally considered vulgar and offensive. It's used as an insult, implying the target is foolish, annoying, or contemptible. The origins of the word are unclear, but its meaning is quite apparent from its crude construction.

It's unlikely you'll find "fartknocker" in formal dictionaries. It falls firmly into the category of vulgar slang. Using this term in most social settings would be considered inappropriate and could cause offense. Because the etymology is not clearly known the origin and use are simply attributed to offensive language.