Also found in: Idioms.
Thoroughgoing; out-and-out: a flat-out promotional campaign; a flat-out deception.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
1. done at maximum speed
2. US downright; complete and utter
US simply; completely and utterly
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
1. using full speed or all of one's resources: a flat-out effort.
2. downright: a flat-out forgery.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.