The flat brimmed baseball hat is said to have originated from Oakland/California.
"The first thing people did when buying a baseball hat in the 90's was bend the brim. Crack and drug dealers would wear glasses and a hat with a straight brim to throw off the stereotypical image" (of a backwards cap or bent baseball cap which was commonly worn in the 1990's)
The style generally moved to the gangster rap scene in the mid to late 90's to associate the rappers with drug dealers, wearing clothes straight off the rack (tags included) and being clean cut and fresh.
Post 2005 people without idea of origin wear the hat as the new style, rarely bending the brim in an "on field" cap style, this style is associated with post 1990 born, any social group from club goers to skaters.
Related is the cultural appropriation of the style and the resentment from a galvanized sub culture, but has come to be featured by many sub cultures during the height of the popularity of the style.
To 'Flatbill' is the act of a man riding shotgun while his girlfriend drives.
Guys flatbilling are often asleep or texting while their girlfriend pumps gas, loads the car with groceries or performs some otherwise useful task. The term comes from the flatbilled baseball-style hats common amongst habitual flatbillers.
Superficial reasons for flatbilling include the lack of a vehicle or a suspended drivers license. Flatbilling can also serve as a power play. In being disinterested and completely unhelpful, the male can manipulate a vulnerable female into a position of subservience. She becomes his chauffeur.
Neither self-respecting men nor self-respecting women will allow themselves to engage in regular flatbilling.
Individuals wearing a fitted, flat-bill hat. If stickers or price tags are still attached, multiple offenses can be expected.
Sources: Rapper, Slang inventor and style originator E-40 mentions in a documentary Blueprint of a self made Millionaire, where the style originated. No gangster rap vidoes before 1994 featured a straight brim hat.
How to wear this hat