Possible Clues:

• In a sour mood
• Like vinegar
• Sardonic
• Heard one from the Balkans is bitter
• Cause célèbre cause of some back-biting!
• Bitter start of a cynical epigram recalling Baudelaire
• Biting, variantly
• "A kindlier touch to her antitheses than pertained to those of that –– damsel" (Mrs A. D. T. Whitney, The Other Girls)
• A European said to be bitter
• Like lemon juice (var.)