The Collaborative International Dictionary of English v.0.48:

Proscribe \Pro*scribe"\, v. t. [imp. & p. p. Proscribed; p.
   pr. & vb. n. Proscribing.] [L. proscribere, proscriptum, to
   write before, to publish, proscribe; pro before + scribere to
   write. See Scribe. The sense of this word originated in the
   Roman practice of writing the names of persons doomed to
   death, and posting the list in public.]
   1. To doom to destruction; to put out of the protection of
      law; to outlaw; to exile; as, Sylla and Marius proscribed
      each other's adherents.
      [1913 Webster]

            Robert Vere, Earl of Oxford, . . . was banished the
            realm, and proscribed.                --Spenser.
      [1913 Webster]

   2. To denounce and condemn; to interdict; to prohibit; as,
      the Puritans proscribed theaters.
      [1913 Webster]

            The Arian doctrines were proscribed and
            anathematized in the famous Council of Nice.
                                                  --Waterland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

proscribed
    adj 1: excluded from use or mention; "forbidden fruit"; "in our
           house dancing and playing cards were out"; "a taboo
           subject" [syn: forbidden, out(p), prohibited,
           proscribed, taboo, tabu, verboten]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PROSCRIBED, civil law. Among the Romans, a man was said to be proscribed
when a reward was offered for his head; but the term was more usually
applied to those who were sentenced to some punishment which carried with it
the consequences of civil death. Code, 9; 49.