always used before a noun
informal
1
chiefly US
: absolute and complete
It was a flat-out lie.
a flat-out refusal
2
: greatest possible : maximum
a flat-out effort
Cite this Entry
“Flat-out.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/flat-out. Accessed 16 Apr. 2026.
Copy Citation
