"In our insular part of Mid-Coast Maine, we used the term "jigaboo" for a scary entity in a ghost story."

That's interesting - there was a thread awhile back about the origins of the song "The Foggy Dew" - not the one about the Easter Rebellion, another one altogther - anyway, the earliest known versions were from the 1600s - and they did not use the phrase, "the foggy dew", but used a word very much like "jigaboo" - "biggeryboo", or something - and it clearly referred to "a scary entity" of the ghost story variety. That term in the song eventually got replaced with "the foggy dew".