adjective
[more ~; most ~]
The form bitterer is almost never used to mean more bitter, but the form bitterest is commonly used to mean most bitter.
having a strong and often unpleasant flavor that is the opposite of sweet
causing painful emotions
A bitter pill or a bitter pill to swallow is something that is difficult or painful to accept.
angry and unhappy because of unfair treatment
feeling or showing a lot of hatred or anger
very cold
to/until the bitter end
until the end of something that may be very bad, unpleasant, etc.
noun
plural -ters
bitters [plural] :a bitter alcoholic liquid that is used especially in mixed drinks
Brit :a type of beer that has a slightly bitter flavor [noncount]