flat-out Share definition Flag flat-out: an adverb that means: at full capacity, at maximum speed, totally, thoroughgoing Ben, pumps working flat-out. A foot of water in the bilge. a flat-out promotional campaign a flat-out deception. flat-out by leocod October 3, 2007 0 0 Get merch Get the flat-out mug. Buy Now — $32.95 See more merch