The Orochi ability is a slang term used by Magic R&D for "when this creature damages another creature tap that creature, it doesn't untap during it's controller's next untap step". The triggered ability was used to define the Orochi snake people of the Kamigawa block
According to Mark Rosewater "[t]he Orochi ability started from flavor. We wanted the snake people (who clearly made sense in green) to have an ability to reflect their venomous attacks. The tap down ability felt like something that would happen if someone bit you with a poison bite. Traditionally tap down creatures rest in white and blue, but green has dabbled with the ability (on cards such Tangle and Elvish Hunter). In addition, the fact that this ability only worked through creature combat helped give the ability a green feel."