bitter
Pronunciation: /ˈbɪtə/
adjective
- 1having a sharp, pungent taste or smell; not sweet: raw berries have an intensely bitter flavour
- (of chocolate) dark and unsweetened.
- 2feeling or showing anger, hurt, or resentment because of bad experiences or a sense of unjust treatment: I don’t feel jealous or bitter she wept bitter tears of self-reproach
- (of a conflict, argument, or opponent) full of anger and acrimony: a bitter five-year legal battle
noun

Phrases
-
to the bitter end
- used to indicate that one will continue doing something until it is finished, no matter what: the workers would fight to the bitter end[perhaps associated with a nautical word bitter denoting the last part of a cable inboard of the bitts, perhaps influenced by the biblical phrase ‘her end is bitter as wormwood’ (Prov. 5:4)]

Origin:
Old English biter, of Germanic origin; related to Dutch and German bitter, and probably to bite