flatly

1. In a flat manner; evenly; horizontally; without spirit; dully; frigidly.

He that does the works of religion slowly, flatly, and without appetite.

2. peremptorily; positively; plainly; forthrightly. «He flatly refused his aid.» Sir P. Sidney.

adverb

In a direct, positive manner: emphatically, flat, positively. Informal flat out. See strong/weak.

The adverb has one meaning:

Meaning #1: in an unqualified manner
  Synonyms: categorically, unconditionally
  Derived from adjective: flat (meaning #3)