"I can't believe you're using a flat bill...you used to be cool!"
Typically a person will adopt terrible posture while flatbilling. Persons 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.
Boyfriend: No way. I'm not flatbilling.
Girlfriend: But you've got the flu!
Boyfriend: Don't care. I can't be seen flatbilling.