OG means “Original Gangster”.
The “OG” in Fortnite is likely a reference to the street gang whose members include rapper 50 Cent. 50 Cent’s father allegedly was a member of the criminal organization, so he claims on one of his tracks and often identifies himself as an OG. The opposition to this might be that it’s just spelled wrong, but I think it is referring to the word origin which was popularized by rap artists in their early years.
In today’s society, gangs are most often male dominated. They promote masculinity over femininity and have more rigid codes of conduct than other types of organizations. When they grow up, some males who come from those environments then join law enforcement agencies or take up other types of traditional leadership roles in society.
Basically this means that Fortnite is intending to portray the OG as powerful, masculine person.
