The phrase "flat denial" means a complete and straightforward refusal to believe or accept something, without leaving any room for doubt or discussion. Full definition
The innocent ought not to suffer for the guilty; each should stand on his own feet and be responsible only for his own deeds — such flat denial of the ideas with which the Old Testament started now became the express teaching of the later Judaism. (religion-online.org)
The mathematical probability forbids flat denial of ET's existence. (firstthings.com)
I am, of course, expecting nothing short of a flat denial of this basic request, possibly with some ad hominems thrown in. (religion.blogs.cnn.com)