It's a reversed cross. It can either mean he's like some kind of anti-christ or that he just doesn't believe in God in general. Since the background of HXH is similar to the real world it's probably a good idea not to go around wearing such a mark when you're trying to look like a normal guy.
he has peter's cross behind his jacket so i think the same thing in his forehead..since peter's cross come in different forms .
and it's a a symbol of humility.
It's called 'the reverse cross' in the prophecy as well as when Killua first saw him from far away.
It's probably not a sign of friendliness. Although he's not afraid to show it, he certainly went out of his way to hide it when he's trying to pretend to be just a normal guy.
As Souhail said, this cross is called St Peter's cross, because St Peter asked to be crucified in a inversed cross as he didn't deserve the same honor as the Christ.
It is a symbol of humility. And yes, the subs do say 'St Peter's cross' in the prophecy.
Coz it's really freaky and screams VILLAIN.
This is St. Peter cross indeed as many said above. He felt he is unworthy of having the same death as christ.
However, as of now it is a satanist symbol.
Shizuku also wears a cross, a pendant. and she's one of the original members of genei ryodan right?