The Collaborative International Dictionary of English v.0.48:

Undue \Un*due"\, a.
   1. Not due; not yet owing; as, an undue debt, note, or bond.
      [1913 Webster]

   2. Not right; not lawful or legal; improper; as, an undue
      proceeding. --Bacon.
      [1913 Webster]

   3. Not agreeable to a rule or standard, or to duty;
      disproportioned; excessive; immoderate; inordinate; as, an
      undue attachment to forms; an undue rigor in the execution
      of law.
      [1913 Webster]

   Undue influence (Law), any improper or wrongful constraint,
      machination, or urgency of persuasion, by which one's will
      is overcome and he is induced to do or forbear an act
      which he would not do, or would do, if left to act freely.
      --Abbott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

undue
    adj 1: not yet payable; "an undue loan" [ant: due]
    2: not appropriate or proper (or even legal) in the
       circumstances; "undue influence"; "I didn't want to show
       undue excitement"; "accused of using undue force" [ant:
       due]
    3: lacking justification or authorization; "desire for undue
       private profit"; "unwarranted limitations of personal
       freedom" [syn: undue, unjustified, unwarranted]
    4: beyond normal limits; "excessive charges"; "a book of
       inordinate length"; "his dress stops just short of undue
       elegance"; "unreasonable demands" [syn: excessive,
       inordinate, undue, unreasonable]

Moby Thesaurus II by Grady Ward, 1.0:

122 Moby Thesaurus words for "undue":
   a bit much, abandoned, aberrant, abnormal, abominable, atrocious,
   boundless, criminal, cutthroat, delinquent, deviant, disgraceful,
   dizzy, egregious, enormous, evil, exacting, exaggerated, excessive,
   exorbitant, extortionate, extravagant, extreme, fabulous, fancy,
   gigantic, gluttonous, gouging, grossly overpriced,
   hardly the thing, high, hyperbolic, hypertrophied, ignominious,
   ill-timed, illegal, immoderate, improper, inappropriate, inapt,
   incontinent, incorrect, indecorous, inept, inequitable, infamous,
   inflationary, iniquitous, inordinate, intemperate, monstrous,
   nonmeritorious, not coming, not done, not outstanding,
   not the thing, off-base, off-color, out of bounds, out of sight,
   out-of-line, outrageous, overbig, overdeveloped, overgreat,
   overgrown, overlarge, overmuch, overpriced, overweening,
   preposterous, prohibitive, sacrilegious, scandalous, shameful,
   shameless, sinful, skyrocketing, spiraling, steep, stiff, terrible,
   too much, towering, unapt, unbalanced, unbridled, unconscionable,
   undeserved, undeserving, unearned, unentitled, unequal,
   unequitable, uneven, unfit, unfitting, unjust, unjustifiable,
   unjustified, unlawful, unmeasurable, unmeet, unmerited, unmeriting,
   unowed, unowing, unreasonable, unrestrained, unrighteous,
   unrightful, unseasonable, unseemly, unsuitable, untimely,
   unwarrantable, unwarranted, unworthy, usurious, wicked, wrong,
   wrongful