Simple past tense and past participle of bitter.(verb)
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Use "bittered" in a sentence
"You may not like Hillary, but is it because of her politics or because you hear the voices of bittered bigoted past saying – Women have no place outside the home?"
"I also need to establish him and as young-and-keen, not particularly seasoned or bittered young man with ideals firmly in place."
"At some point in the not-too-distant past there was one style: India Pale Ale and it meant, basically, one thing: a strongly bittered bite in the finish."