Green’s Dictionary of Slang

(US) used to express stupidity e.g. nerf-brained adj., stupid.

1984		‘Savage’ Steve Holland Better Off Dead [film] You’re a spastic nerfbag all the time [HDAS].
1986		L. Sliwa Attitude 29: Their role models are the nerf-brained, pearly toothed, peroxide-haired babies depicted in the media.
2003		K.L. Henscheid ‘The Argument Story Part II’ in Star Wars Stories 🌐 ‘Well, just lay it on me, nerf-brain!’ Jenna shouted at him.