W00! is an Internet-based slang word used to express joy and excitement. It is synonymous with "hooray" or "yippee!"
It may be expressed as woot, woot!, w00t, woo7, woo+, or with multiple os (woooooot). The word is most frequently used with zeros for os, and may date back to the 1990s. At some point the word was transformed to its current incarnation by way of the l33t language (pronounced "leet", rhymes with "meet"), an Internet-based form of speaking, where letters are sometimes replaced by numbers.
W00t, Urban Dictionary