Definition of “bitterly” - Learner’s Dictionary

bitterly

adverb uk us /ˈbɪtəli/
UPSET

in a way that shows strong negative emotion such as anger or disappointment:

We were bitterly disappointed about the decision.
Greg complained bitterly about the conditions.
WEATHER

If it is bitterly cold, the weather is extremely and unpleasantly cold.

(Definition of “bitterly” from the Cambridge Learner's Dictionary © Cambridge University Press)