Green’s Dictionary of Slang

in the music business, a fan of loud, monotonous, ‘heavy metal music’, usu. a youth who plays a make-believe (or even cardboard) guitar and shakes his head violently as he watches or listens to his heroes.

1985		‘Jennifer Blowdryer’ Modern English 18: headbanger (n): Describes the pleasure of rabid fans, tossing their shag cut back and forth to the sound of heavy metal music.
1998		(con. 1986) G. Pelecanos Sweet Forever 75: He liked the crowd too — not all the way headbanger or absolute punk.
1999		Guardian 28 June 7: Why these tiresomely right-on head-bangers and not, say, the delightful Catatonia?
2001		C. McPherson Port Authority 1: She tended to go out with headbangers.