Green’s Dictionary of Slang

1. straightforward, unadorned, blunt, esp. of speech.

1901		‘Miles Franklin’ My Brilliant Career 15: Her feelings being much more defined, it was amusing to hear the flat out opinions she expressed to Mr Blackshaw.
1906		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:ii 136: flat-out, adj. Frank, direct. ‘Didn’t I ask you the flat-out question, whether you were there?’.
1996		G. Pelecanos (con. 1949) Big Blowdown (1999) 146: Boyle knew that was a flat-out lie.

2. complete, utter.

1944		Yorks. Post 14 Mar. 2/7: It was faced with the alternatives of a flat-out assault straight at the main defences.
1972		Eble Campus Sl. Oct. 2: flat out Adj. – complete; Mary’s a flat-out failure.
1978		R. Charles Brother Ray 277: I’m as scared of dying as anyone else [...] It’s one of those constants—a flat-out certainty—that you can’t escape.
1999		Indep. on Sun. Culture 22 Aug. 2: I often receive [...] displays of flat-out anxiety from those near me.
2007		C. Goffard Snitch Jacket 182: He was one of our favorite freaks, a flat-out genius.
2009		L. McIlvanney All the Colours 103: A flat-out punt on a story.