Bitterness is the hallmark of many poisonous plants. It is the warning flavour that signals something unsavoury is about to happen. Jennifer McLagan writes in her cookbook, Bitter:
A reluctance to eat bitter foods is understandable, as we all have an innate aversion to bitter tastes. Our tongues are covered in taste buds that are very adept in detecting even the smallest traces of bitterness. This is a natural defense system to protect us: many poisons are bitter, so our response when tasting something very bitter is to grimace and often to spit it out. (1)
In its adjectival form, bitter is related to taste, as in a flavour that is pungent and not sweet. It is also connected to feelings of lingering resentment due to a sense of real or perceived mistreatment or of being unjustly maligned. Sprinkled throughout our idioms, bitter wars are fought, bitter pills are swallowed, the bitter must always be taken with the sweet, and inevitably, some people, events or relationships come to a bitter end.
In the Oxford Dictionary under the category of similar words, the following are listed:
(1) McLagan, Jennifer. Bitter (p. 2). Potter/Ten Speed/Harmony/Rodale.