副词

1. 强烈地，激烈地，极为（愤恨、憎恶等）
You use bitterly when you are describing an attitude which involves strong, unpleasant emotions such as anger or dislike.

1. with bitterness, in a resentful manner

e.g. she complained bitterly

2. indicating something hard to accept

e.g. he was bitterly disappointed

3. extremely and sharply

e.g. it was bitterly cold
bitter cold

Synonym: piercinglybitinglybitter