Dic: flat (some of its meanings as an adjective)

• The Foreign Ministry has issued a flat denial of any involvement.
• 'Whatever you say,' he said in a deadly flat voice. `I'll sit here and wait.'.
• Her voice was flat, with no question or hope in it.
• 'I know you,' he said flatly, matter-of-fact, neutral in tone.

14. ADJ : ADJ n | A flat denial or refusal is definite and firm, and is unlikely to be changed.

19. ADJ | You use flat to describe someone's voice when they are saying something without expressing any emotion.

... Cobuild

Cf.

5. Free of qualification; absolute: a flat refusal.

12. Unmodulated; monotonous: a flat voice.

13. Lacking variety in tint or shading; uniform: "The sky was bright but flat, the color of oyster shells" (Anne Tyler).

.... The American Heritage