The Collaborative International Dictionary of English v.0.48:

Involuntary \In*vol"un*ta*ry\, a. [L. involuntarius. See In-
   not, and Voluntary.]
   [1913 Webster]
   1. Not having will or the power of choice.
      [1913 Webster]

   2. Not under the influence or control of the will; not
      voluntary; as, the involuntary movements of the body;
      involuntary muscle fibers.
      [1913 Webster]

   3. Not proceeding from choice; done unwillingly; reluctant;
      compulsory; as, involuntary submission. Involute

WordNet (r) 3.0 (2006):

involuntary
    adj 1: not subject to the control of the will; "involuntary
           manslaughter"; "involuntary servitude"; "an involuntary
           shudder"; "It (becoming a hero) was involuntary. They
           sank my boat"- John F.Kennedy [syn: involuntary,
           nonvoluntary, unvoluntary] [ant: voluntary]
    2: controlled by the autonomic nervous system; without conscious
       control; "involuntary muscles"; "gave an involuntary start"
       [ant: voluntary]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "involuntary":
   at odds, automatic, averse, binding, blind, casual, compulsive,
   compulsory, conditioned, cursory, dictated, differing, disagreeing,
   disinclined, disobedient, forced, fractious, gut, ill-advised,
   ill-considered, ill-devised, imperative, impulsive, inadvertent,
   indeliberate, indisposed, indocile, inevitable, inherent, innate,
   instinctive, instinctual, libidinal, mandatory, mechanical,
   mutinous, natural, necessary, obligatory, opposed, perfunctory,
   recalcitrant, reflex, reflexive, refractory, required, resistant,
   snap, spontaneous, subliminal, sulky, sullen, unadvised,
   uncalculated, unconscious, unconsenting, unconsidered,
   uncontrollable, uncontrolled, undeliberate, undeliberated,
   undesigned, unguarded, unguided, unintended, unintentional,
   unlearned, unmeant, unmeditated, unplanned, unpremeditated,
   unprompted, unpurposed, unstudied, unthinking, unthought-of,
   unwilled, unwilling, unwitting



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

INVOLUNTARY. An involuntary act is that which is performed with constraint,
(q. v.) or with repugnance, or without the will to do it. An action is
involuntary then, which is performed under duress. Wolff, Sec. 5. Vide
Duress.