The term Google Ninja was first coined by an SEO company called SEORM in reference to both a collaboration site they created to allow SEO professionals to help each other, as well as a general description for a highly skilled SEO engineer who seems to have the inside track on ranking sites within google.
Google Ninja later became a certification standard inside the SEO community, obtained by completing a course with SEORM. Achieving Google Ninja certification is a great way to show potential employers and or clients that your SEO skills are truely formidable and set you apart from the Google Ninjanabe's (ninja wannabe's) and Google Nonja's (non-ninjas).
Holders of the coveted Google Ninja certification are typically given membership to a private forum and site reserved only for people holding the certification or for premium account holders who pay a monthly fee. These forums and sites contain a vast knowledge base of current SEO tactics and Tools that help the Google Ninjas keep an edge over the competition.
Did you see that guys googlefu? Dude must be one hell of a google ninja.