Learn That Word

Synonyms for Bitterness (same or very similar meaning)

WordNet sense 1 (a feeling of deep and bitter anger and ill-will):
gall, rancor, resentment, rancour

WordNet sense 2 (the taste experience when quinine or coffee is taken into the mouth):

WordNet sense 3 (a rough and bitter manner):
acerbity, acrimony, jaundice, tartness, thorniness

From the ODE community, based on WordNetadd/edit