Boo

 People have used "boo" to startle others since the mid 16th century, but the word didn't make it into the ghost vernacular until less than two centuries ago. The word's origins are unclear but a few hundred years ago it was used to announce your presence, not to frighten others. For instance the old Scottish proverb, "He can't say bo to a goose", meaning he was too timid to make himself known. In 18th century Scotland boo started being used for scary stuff, things that go bump in the night like the bogeyman. From that point the once well spoken ghosts were reduced to saying, "boo". Many other Halloween traditions such as carving pumpkins also came from Scottish immigrants. Immigrants enrich our lives in many ways.