adjective

1. designating or having a sharp, often unpleasant taste; acrid, as quinine or peach stones
2. causing or showing sorrow, discomfort, or pain; grievous
3. sharp and disagreeable; harsh; severe; piercing a bitter wind
4. characterized by strong feelings of hatred, resentment, cynicism, etc.

Etymology: ME < OE biter, akin to bītan, bite

adverb

1. in a way that is bitter; bitterly
2. extremely it was bitter cold

noun

1. a bitter quality or thing take the bitter with the sweet
2. Brit. bitter, strongly hopped ale

transitive verb, intransitive verb