The Collaborative International Dictionary of English v.0.48:

Inadmissible \In`ad*mis"si*ble\, a. [Pref. in- not + admissible:
   cf. F. inadmissible.]
   Not admissible; not proper to be admitted, allowed, or
   received; as, inadmissible testimony; an inadmissible
   proposition, or explanation. -- In`ad*mis"si*bly, adv.
   Inadvertence

WordNet (r) 3.0 (2006):

inadmissible
    adj 1: not deserving to be admitted; "inadmissible evidence"
           [ant: admissible]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "inadmissible":
   adrift, beside the mark, beside the point, beside the question,
   ethnocentric, exceptionable, exceptional, excluding, exclusive,
   exclusory, extraneous, extrinsic, ill-adapted, ill-assorted,
   ill-chosen, ill-favored, ill-fitted, ill-matched, ill-sorted,
   ill-suited, ill-timed, immaterial, impertinent, impossible,
   improper, inapplicable, inapposite, inappropriate, inapt,
   incidental, inconsequent, indefensible, inept, infelicitous,
   insular, intolerable, irrelative, irrelevant, mal a propos,
   maladjusted, malapropos, misjoined, mismatched, mismated,
   misplaced, narrow, nihil ad rem, nonessential, not at issue,
   objectionable, off the subject, out of character, out of joint,
   out of keeping, out of line, out of place, out of proportion,
   out of season, out of time, out of tune, out-of-the-way,
   parenthetical, parochial, preclusive, prescriptive, preventive,
   prohibitive, restrictive, seclusive, segregative, select,
   selective, separative, snobbish, unacceptable, unadapted, unapt,
   unbecoming, unbefitting, undesirable, unessential, unfit, unfitted,
   unfitting, unqualified, unseasonable, unseemly, unsuitable,
   unsuited, untenable, untimely, unwanted, unwelcome, xenophobic



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

INADMISSIBLE. What cannot be received. Parol evidence, for example, is
inadmissible to contradict a written agreement.



The Devil's Dictionary (1881-1906):

INADMISSIBLE, adj.  Not competent to be considered.  Said of certain
kinds of testimony which juries are supposed to be unfit to be
entrusted with, and which judges, therefore, rule out, even of
proceedings before themselves alone.  Hearsay evidence is inadmissible
because the person quoted was unsworn and is not before the court for
examination; yet most momentous actions, military, political,
commercial and of every other kind, are daily undertaken on hearsay
evidence.  There is no religion in the world that has any other basis
than hearsay evidence.  Revelation is hearsay evidence; that the
Scriptures are the word of God we have only the testimony of men long
dead whose identity is not clearly established and who are not known
to have been sworn in any sense.  Under the rules of evidence as they
now exist in this country, no single assertion in the Bible has in its
support any evidence admissible in a court of law.  It cannot be
proved that the battle of Blenheim ever was fought, that there was
such as person as Julius Caesar, such an empire as Assyria.
    But as records of courts of justice are admissible, it can easily
be proved that powerful and malevolent magicians once existed and were
a scourge to mankind.  The evidence (including confession) upon which
certain women were convicted of witchcraft and executed was without a
flaw; it is still unimpeachable.  The judges' decisions based on it
were sound in logic and in law.  Nothing in any existing court was
ever more thoroughly proved than the charges of witchcraft and sorcery
for which so many suffered death.  If there were no witches, human
testimony and human reason are alike destitute of value.