Source : Webster's Revised Unabridged Dictionary (1913)

Imbitter \Im*bit"ter\, v. t. [imp. & p. p. {Imbittered}; p. pr.
   & vb. n. {Imbittering}.] [Pref. im- in + bitter. Cf.
   {Embitter}.] [Written also {embitter}.]
   To make bitter; hence, to make distressing or more
   distressing; to make sad, morose, sour, or malignant.

         Is there anything that more imbitters the enjoyment of
         this life than shame?                    --South.

         Imbittered against each other by former contests.
                                                  --Bancroft.