ADVERB
1-in a way that shows no emotion or interest
2- in a complete, very clear, or certain way
Synonyms: anemic
apathetic
apathy
aridity
blah
half-hearted
half-heartedly
hell
incurious
nonchalance
nonchalant
nonchalantly
nonconcern
passionless
unimpressed
https://dictionary.cambridge.org/us/dictionary/english/flatly
IPA transcription: /ˈflæt.li/
"I've got training to do", Annabeth said flatly. (p93)
The witness responded flatly to the judge's questions.
He flatly denied that he saw what had happened.
ETMOLOGY
Early 15c
in a literal sense, from flat (adj.) + -ly (2). Meaning "in a plain manner" is from 1560s; sense of "in a dull manner" is from 1640s.
https://books.google.com/ngrams/graph?content=flatly&year_start=1800&year_end=2019&corpus=en-2019&smoothing=3