1. in a direct and definite way, without hesitation or doubt
2. in a dull or uninteresting manner, without emotion or enthusiasm
3. in a flat position or manner, without curves or slopes

Examples

• She flatly refused to help with the project.
• The manager flatly denied the accusations against him.
• He spoke flatly, showing no emotion about the news.
• The actor delivered his lines flatly, boring the audience.
• The book lay flatly on the table.
• The road stretched flatly across the desert.
• I flatly disagree with your assessment.