The Collaborative International Dictionary of English v.0.48:

Involuntarily \In*vol"un*ta*ri*ly\, adv. [From Involuntary.]
   In an involuntary manner; not voluntarily; not intentionally
   or willingly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

involuntarily
    adv 1: against your will; "he was involuntarily held against his
           will" [ant: voluntarily]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "involuntarily":
   automatically, blindly, by reflex, carelessly, compulsively,
   heedlessly, inadvertently, inconsiderately, indeliberately,
   instinctively, malgre soi, mechanically, reflexively,
   thoughtlessly, unadvisedly, uncalculatedly, unconsciously,
   under compulsion, undesignedly, unexpectedly, unintentionally,
   unpremeditatedly, unreasoningly, unthinkingly, unthoughtfully,
   unwillingly, unwittingly, without design, without premeditation,
   without willing