This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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v. Simple past tense and past participle of imbitter.

"The pride of ancestry, which rankled in the bosom of the ancient gentry, was more openly expressed by their ladies, and was, moreover, imbittered not a little by the political feuds of the time, for most of the Southern chiefs were friends to the authority of the Queen, and very jealous of the power of Murray."