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A Boo is a type of ghost featured in Mario video games. Boos made their first appearance under the name of "Boo Diddly"(a play on Bo Diddley an early R&Bguitarist) in Super Mario Bros. 3 and was virtually invincible to all attacks with the exception of the Starman and the Hammer Bros. Suit.
"The Boo's in Paper Mario are soooooo cute!"
Boo by TheBlackMew September 7, 2006
Related Words
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1: An affectionate name for someone you love.

2: Something that a ghost says.

3: The short version of my cat's name, Boo-Boo.
1: I like cuddling with my boo.

2: BOO!

3. "Boo, get out of the dishwasher!"
boo by Loki August 24, 2003
Word used when expressing your disdain when somebody changes their mind about something, i.e. after something improves and is worthy of their notice.
You didn't say boo to the White Sox until they won the world series.

That guy never said boo to you until you lost all that weight.
boo by Paddlegurl December 28, 2005
A term of endearment, like baby.
"I'll be in here, boo" -- Method Man (You're all I need 1994).
boo by VAKI5 September 14, 2003
The word a ghost says to you to say "hi!"
Ghost: Boo!
Me: How are you?
boo by Ghost33 August 16, 2009