What does "flat"mean in this sentence? "Any suggestion that this was an insult to the United States is just flat wrong." What does "flat"mean in this sentence?

What is the Bloody Eagle?

First notebook entry: Introducing myself l love english! at first i love listening to the english songs,so i want to learn english to know what the songs lyrics means.as time gone by,i found english is a way to know the world outside and you want to go abroad?you must learn english!