1. someone who acts retarded.
2. someone who acts spaztic and out of control.
3. someone who does something stupid.
This word is a comparison to children with disabilities or uncontrollable movements who are required to wear helmets to prevent head injuries.
Hey, Helmet Boy over here thought that light beer meant that it has less alcohol in it!
HB - "Lemmego! Lemme go!" "Chill out helmet boy! you'll get your turn..."