In… flat Definition 1 of 3Pro Tip 1/3Rapid Action Emphasizes the speed of an action or reaction, suggesting something happened quickly. After hearing the news, she decided in flat to help. Pro Tip 2/3Emotional Intensity Shows a strong, sudden emotion like falling in love or getting angry. He goes in flat when he sees a piece of artwork he loves. Pro Tip 3/3Idiomatic Usage Used in informal settings; may not be understood in formal or non-idiomatic contexts. When she plays poker, she always goes in flat.