There are several synonyms for the phrase "bitterness of spirit." It refers to a feeling of resentment, anger, or disappointment that is long-lasting. One possible synonym is "rancor," which implies a deep-seated animosity that is hard to overcome. Another option is "acrimony," which suggests a harsh or biting quality to the bitterness. "Spite" is another synonym, indicating a desire to harm or undercut others due to one's own bitterness. Finally, "malice" may also be used to describe this state of mind, as it suggests a deliberate desire to inflict harm or suffering on others as a result of one's own bitterness.