A scary ghost creature in the Mushroom Kingdom. Mario faces these a lot. These pearly-white ghosts will become invisible if anyone faces them. They usually cannot be disposed of. There are also other Boos, like the Big Boo, King Boo, and more.
"I thought I heard a Boo, but I turned around and saw nothing."
boo is a term that is derived from the French word "beau" meaning beautiful. In 18th century England it meant an admirer, usually male. It made it's way into Afro-Caribean language perhaps through the French colonisation of some Caribean islands.