boo 

expression of dissapproval.

B. Gates: This is the all-great Windows XP.

Me: boo!

boo by vin July 22, 2004

boo 

Sound that a fake ghost would make

Ghost:BOOOO!
Child:AHHHHHHH!

boo by BEYONCE November 8, 2003

boo 

A word used to scare people.

Guy: "BOO!"
Guy2: "omgwtfbbq"

boo by Gracin July 29, 2005

boo 

A word used to scare a person

Person 1: BOOOO!!!
Person 2: AHHH!!!

boo by Reagan Lausche February 21, 2005

Boo 

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 by SeaCaptainDrake December 6, 2003

Boo 

A ghetto term of endearment for your significant other. Possibly for your baby momma.

"Heather is my Boo, now and fo-eva fo-sho"

Boo by Patricia Reyes December 14, 2008

boo 

A guy steps on home plate.
The crowd doesn't like that guy.
They shout "BOO".

"Boo, you stink a lot you bad player."

boo by medium bro December 6, 2006