The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Is there anything that more imbitters the enjoyment of
         this life than shame?                    --South.
   [1913 Webster]

         Imbittered against each other by former contests.
                                                  --Bancroft.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Embitter \Em*bit"ter\, v. t.
   To make bitter or sad. See Imbitter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

embitter
    v 1: cause to be bitter or resentful; "These injustices
         embittered her even more" [syn: embitter, envenom,
         acerbate]