James Cameron -- Attention Nerds: I Didn't Steal 'Avatar'

James Cameron is sick and tired of people claiming he stole the concept behind "Avatar" -- and has now filed legal docs, asking a judge to dismiss a $2 billion lawsuit that says the movie wasn't his idea.

TMZ broke the story ... science fiction writer and professional nerd Bryant Moore sued Cameron for $2.5 billion back in December -- claiming Moore came up with the basic premise behind "Avatar" ... and Cameron jacked it.

In his lawsuit, Moore says Cameron pilfered several ideas from Moore's screenplays -- including bioluminescent flora/plant life, unbreathable atmospheres, matriarch support of hero vs. heroine, and spiritual connections to environment ... and reincarnation ... and .... mist.

Now, Cameron's fighting nerd fire with nerd fire -- insisting the Na'vi (the alien race in 'Avatar') are NOTHING like the Gavadeen Alliance or the Pollinators (the alien races in Moore's movies).

In fact, Cameron says NOTHING about Moore's scripts has anything to do with "Avatar" -- they're different from beginning to end.