Synonyms of flat-out

1 Syllable
frank
2 Syllables
promptly, frankest, utter, straight-out, utters, all-out, swiftly, quickly, franker, fleetly
3 Syllables
snappily, plainspoken, heart-to-heart, unbounded, punctually, all-fired, hastily, posthaste
4 Syllables
unrelieved, lickety-split
5 Syllables
unrestrainedly, expeditiously
7 Syllables
unpremeditatedly