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1.
The gay ass way that people wear their hats nowadays. They even leave on the gay ass round shiny stickers that tell you how small the casing for your brain is. You used to shape your bill into a curved thing that would fit snugly on your head...back when it was cool to wear a hat.
"Damn foo, look at all those wannabee gangsta's flatbillin it!"

"I can't believe you're using a flat bill...you used to be cool!"
by therealMarcos May 02, 2006
 
2.
n. - when a baseball hat is worn with an unbent bill. Often sported by wannabe thugs and douchebags who play the sport of baseball.
"I thought I saw K-Fed on campus today, but it was just a baseball player rockin' a flatbill."
by diegosoulmate December 17, 2008
 
3.
Flatbill- (v) the act of a man riding shotgun while his girlfriend drives.

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.
Girlfriend: It's alright, I'll drive.
Boyfriend: No way. I'm not flatbilling.
Girlfriend: But you've got the flu!
Boyfriend: Don't care. I can't be seen flatbilling.
by Flatbiller3000 November 12, 2011
 
4.
A hat with the flatest of brims...not an ordinary baseball cap. Gives user high amounts of swag.
Look at that bro over there, he has so much swag in his flat bill.
by Chez. December 28, 2011