A person who wears a cock-eyed ball cap with a perfectly flat bill perched askew on one's head. Pejorative.
A fad originally started by black "urban youth" which denoted prosperity (because new hats have unformed bills). Leaving the price tags and labels on was common as well, indicating that the wearer could afford a new hat. This falls under the social phenomena of "nigger rich".
Like most "street fashion", this style has largely been appropriated (or stolen) by privileged white hipsters who want to express solidarity with black urban youth, and indicate that they too are "hard" and "from the streets".
"It's so ironic to see so many lily-white flat billers cranking hip-hop in their monster trucks and jeeps."
"Why do Flat billers even bother to wear caps? They fall off their head when they lean over and they definitely don't keep the sun out of their eyes... Maybe it's to keep their ear from getting sunburned when they are driving?"