A High Pitched Battle Cry Used by the best fighters, or certain martial artists. Two Well Known fighters and manly badasses who use this shout are Bruce Lee, and Kenshiro, of the Anime, Fist Of the North Star.
It strengthens attacks, pwns enemies, and makes you look badass. if you repeat ATA, as ATATATATATA!!!! then you're probably throwing rapid barrages of punches at some poor motherfucker.
It also variates into "WATA!!!" "ATOU!" and various other similar shouting.