Green’s Dictionary of Slang

1. exhausted.

1918		C.J. Dennis ‘Dad’ in Digger Smith 29: Dad is ill an’ took to bed, / Flat out with work.
1947		D. Davin For the Rest of Our Lives 374: ‘How were they feeling at Brigade?’ ‘Flat out when I left.’.
1951		Cusack & James Come in Spinner (1960) 38: ‘Tired, dearie?’ ‘Flat out.’.
1998		R.G. Barrett Mud Crab Boogie (2013) [ebook] It [i.e. an automobile] [...] looked like an absolute shithouse and mechanically would have been flat out pulling a French letter off a slack dick.

2. busy.

1941		N.Y. Herald Trib. 29 June 9/3: If you’re up to your ears in work or doing anything as hard and fast as you can, you’re ‘flat out,’ which finds its origin in Australia’s favorite sport — horse racing.
1951		D. Stivens Jimmy Brockett 25: All the next day, Friday, I was flat out like a possum up a gum-tree. Jimmy Brockett had his irons in the fire all over the place.
1955		J. Morrison Black Cargo 155: I’ve been going flat out like a lizard since eight o’clock this morning.
1965		W. Dick Bunch of Ratbags 302: I’ve been flat out with cases until ten o’clock every night this week.
1977		in K. Gilbert Living Black 37: They’re flat out to get a few bob in for Friday night at the club.
1987		M. Bail Holden’s Performance (1989) 348: We’ve been flat out lately.
2000		Guardian G2 17 Feb. 14: We are flat out trying to keep up with the demand.

3. hard put.

1993

Smith & Noble Neddy (1998) 221: She had [...] come straight to Sydney, bringing with her a loaded pump-action shotgun to protect me if the need arose. [...] You would be flat out finding too many blokes with as much go in them as my Debra had.