Though sources place the origins of the word as from the Latin verb boare (Greek boaein), "to cry aloud, roar, or shout", it's a slightly strange word, only used in this way as an interjection.
Like my very first post on this blog, about onomatepoeias and how they differ around the world, it's interesting to see how expressive sounds like "boo" differ from country to country.
Surprisingly, most European countries retain the "boo" sound, though spelling differs slightly. In Spanish and Portuguese it's "bú", German "buuu", French "bouh", Dutch "boe", Swedish "bu", Norwegian "bø", and Danish "bøh".
However, in Asian countries the sound differs. In Japan, "buu" is the sound a pig makes - our "boo" is "waa" to them. In Laos and Thailand they say "ja", and in China they say "pēi".