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1.
An Australian slang word meaning to go at the maximum speed possible. To push yourself or your vehicle to the extreme of velocity.
"Wow, that car is going flat-chat! It's out of control!'
by Sir Glame November 11, 2010
 
2.
At full speed, going as fast as you possibly can. (Australian)
"We were fully gunning this old HQ Kingswood totally FLATCHAT downhill with a tailwind, and we still couldn't get away from the copper on his pushie."
by Alex From Oz October 04, 2007
 
3.
When someone is extremely busy
He is flat chat on other work today and will get back to you tomorrow.
by odelay February 23, 2012