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1.
Used to show extreme passion and expression towards anyone or anything. Usually follows an exciting event or victory. An experienced user of the phrase may say it at random out of pure addiction to the word.

"Hooooooooo!"
(After winning in street fighter) "Hoooooooo!"
"Wezy Hooooooo!"
"Last night I fucked her in the ass doggystyle! Hooooooo!"
by Voorhees 433 February 28, 2006
 
2.
A battlecry, often used out of content of a battle to signify great excitment about a subject, usually used with the subject before the cry itself, originally from "Thundercats". The number of 'O's is up to you.
Thundercats, HOOOOOO!!!!

Lunch HOOOOOOOOOOO!!!
by Gushon May 30, 2012