Where on earth did "Boo" ... come from?

On Hallowe'en, it comes from goblins.

The "Boo" of goblin lore preceded the "Boo" of disapproving audiences.

Goblins used to say "Boo" to scare people away. After a while, people lost their fear of goblins so they started saying "Boo" to scare the goblins away.

"Boos" seldom scare anyone away anymore, especially politicians.