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1.
In the anime(term for Japanese cartoons) and/or manga(Japanese Comics)called Naruto,two of the charecters are Sasuke Uchiha,the rival of Naruto(the main charecter),and Itachi Uchiha,brother of Sasuke and the one he aims to kill.SasuIta(or ItaSasu)is short for Sasuke X Itachi.It's ment to indicate Sasuke and Itachi as being a couple or having an intimate relationship.It can be used in describing what a fanfic(written story made by a fan) is about,what a doujinshi(comic based on a manga/anime and made by a fan or a comic based on the authors own original ideas)is about or the theme of any similar thing involving Naruto charecters.
When asked about who your favorite dating couple from Naruto is,you can reply:"I like SasuIta the best."-saying that you think Sasuke and Itachi together.
When asked what your site is about, your reply can be:"It's mainly about SasuIta."-indicating that your site's theme is based on Itachi and Sasuke as a couple.It's the same when saying that your story has SasuIta in it, meaning that a featured couple is Sasuke and Itachi.
by My name is Chikiru July 07, 2006
 
2.
A slash pairing from the anime Naruto involving the brothers Sasuke and Itachi Uchiha. Itachi is six years older than Sasuke, and Sasuke has lived his life for the sole purpose of killing Itachi for revenge after he slaughtered their clan.
SasuIta refers to Sasuke as the seme; the dominant one; the one 'on top' and Itachi as uke; the submissive one; the partner 'on the bottom'. It is a large deviation from the canon relationship between the brothers because in SasuIta Itachi is normally very weak or submissive and in the anime he is very strong and has the ability to easily kill Sasuke.
Oh my god; SasuIta is the hottest thing I've ever read. Uke Itachi is the stuff of dreams!
by DeadAtTwentyTwo October 19, 2009