The first meaning is another name for the head
of the penis
. Due to the similarity of shape and positioning on the end of the penis.
However you can also call someone a 'helmet'. You would use this if they are acting like a total idiot
by saying or doing ridiculous
"we fucked all night, and now my helmet is red raw
"I'm sick of him always pulling his balls out in front of everyone when he's drunk- he acts like a total helmet!"