Helmet hang is when the male genitalia is displayed and is considered unsightly and grotesque. A guy with to much helmet hang will find it almost impossible to get laid.
Girl 1:- what's that new guy like that your dating?
Girl 2:- he's cute but he has really bad helmet hang!
Basically, you laying there dead on the pavement with a white chalk outline of your dead carcass due to a large caliper weapon(like a .44) used to blow your brains out with one hollow tip. Leaving you lying there for everyone to see with your Helmet (slang obviously for head) Hung or Hanging in a Hung position.
Gangster #1 -"Holy Shit Blood, look at that dam cat over there chalked out in white!"
Gangster #2 "Oh shit, his helmet is def hangin! Yo, it must of been a "fo fo" for sho!" Now that's a Helmet Hang if I ever seen one!!!