From RGDWiki

NNGC (referred to as "NextGen" in some documents; might mean "Next Gen Nintendo GameCube" or "Next Gen Game Console" or similar) was an internal codename for the Wii project, used before the "Revolution" (RVL) name was decided on.[1] It is referenced in various places, including the names of the NDEV USB drivers and game files of The Legend of Zelda: Twilight Princess. It has been theorized that the NDEV's full name is "NNGC Dev". The name was first seen in late 2004 in the earliest planning documents for the system.

According to SDK header files, the NNGC name fell out of use around July 2005, and files named "NNGC" were changed to Revolution or RVL. However, some documents continued to use the name into 2006.