I’m not sure that it means anything in the Sumerian language. The name was first used by Zechariah Sitchin , who developed the idea based on Sumerian and Babylonian and even Biblical mythology about a doomsday Planet X. There is a LOT of fascinating material out there, none of it true, though, based on the writings of Zechariah Sitchin and several others. MY understanding of the Niberu mythos is that our sun, Sol or Helios, has a twin or double star that makes a strange elliptical orbit from way out there to where we cannot detect it, and comes closest to our sun in an elliptical orbit somewhere between Mars and Jupiter. Sometimes it is close enough to Earth to wreck apocalyptic havoc, and may be responsible for a planet between Mars and Jupiter getting destroyed and becoming the asteroid belt. This twin star is perhaps a brown dwarf about the size of our Ice giants Neptune and Uranus,, and is about 12x the density of Jupiter, has its own planets—including one that the Ancient Astronauts the Annunaki, came from. There is a ton of material out there, but the place to start is with Sitchin and the Nibiru cataclysm. Like my view about U.F.O.s there is a lot of good stuff but I don’t believe it personally