Def. expressing joy (it could be after a triumph, or for no reason at all); similar in use to the word “yay”
w00t was named the word of the year in an Online poll done by a very well know dictionary. This year’s winning word first became popular in competitive online gaming forums as part of what is known as l33t (”leet,” or “elite”) speak—an esoteric computer hacker language in which numbers and symbols are put together to look like letters. Although the double “o” in the word is usually represented by double zeroes, the exclamation is also known to be an acronym for “we owned the other team”—again stemming from the gaming community.
Source: Urban Dictionary
History: The current-day use of the word w00t stems from hackers in the early to mid 80’s. While communicating with each other groups of hackers such as Razor1911 would need lingo which nobody else would be able to understand to express milestones in their hacking. One such milestone was gaining root access, but the term rooted or “gained root access” was easily understood so the term was changed to w00t to help disguise. Because of the difficulty of “rooting” many times the term w00t would be much in a celebratory tone. It later evolved to simply be a celebratory remark rather than a hacking milestone.