Derives from an individual who likes saying random things - elderberry comes from one time use in monthy python and fish are random in all ocasions. Can be used to describe a very rare or strange thing, or as a random statement.
Dude you smell of elderberryfish!
Well maybe I'll go ad find an elderberryfish.
Sure, and elderberry fish live in abundance on mars (!)
Dude, how many people have ever heard the word Elderberry fish before!