Yatta is defined: Japanese short form for "yarimashita" meaning "did it". When used as an exclamation Yatta! it has the meaning of "I did it!" -From the Anime Encyclopedia.Why is this important? It's the title of a popular song in Japan, performed by six men who wear nothing but (ahem) fig leaf-adorned underwear.
There are some pictures at that site, for those who are broadband-impaired .... but they really don't do justice to it.