Japanese short form for 'yarimashita' meaning "did it".
When used as an exclamation 'Yatta!' it has the meaning of "I did it!"
There's also a merry ditty by Japanese comedy group Happa-tai (Leaf-team) with this name. A video of a performance of this has been making its way around the internet. It seems to be popular as it depicts the performers prancing around in small white shorts with pictures of fig leaves on the front, reaffirming the popular view that the Japanese are truly mad.
To find: Do a google search for "yattapv.asf".
PS: There is a flash movie floating around based on the video. It lives on E2 here.
There's also a West Bank town with this name, but that's another node