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1.
Racial slur aimed at asians and pacific islanders. Most often used by the small minded and insecure to make them feel superior to someone. Frequently used with or in place of chinc or slant eyes as an adjective to a more universally used slur like bastard or fucker.
That flat faced bitch wrecked the curve and turned my D into an F!
by so what November 01, 2005
 
2.
A bear's term for a human.
Bear1: My cub was shot by a flat-face ...

Bear2: A flat-face put my cub in a zoo.

Bear3: Flat-faces bring nothing but trouble. Hmph.
by AnalPenetrationByForce October 10, 2011
 
3.
A slang term used by the Canadian military to describe civilians.
Hey Flat Face, how are you?

He retired, he is just a flat faced civy now.
by Leopardmech October 18, 2012
 
4.
Derogatory term regarding those suffering from mental retardation (particularly, Down Syndrome). In reference to the flatness of their faces.
Ben: Oi! Mate! Did you see that Flat Face?
Tim: Yea, he was a right mong!
by Bedun August 20, 2011