形容詞 involuntary:

1. not subject to the control of the will- John F.Kennedy
   同義詞：nonvoluntary, unvoluntary
2. controlled by the autonomic nervous system; without conscious control

1. An involuntary of joy escaped from her lips.
   她那情不自禁的喜悅脫口而出。
2. A sudden, involuntary, spasmodic muscular contraction causing severe pain, often occurring in the leg or shoulder as the result of strain or chill.
   痙攣一種由于緊張或寒冷而導致的肌肉突發(fā)性、無意識的、間歇性的緊縮，能導致劇烈的疼痛，通常發(fā)生在腿部或肩部
3. The involuntary and consistent response of an organism to a given stimulus.
   生理反應生物體對所受到的刺激作出的無意識的連貫反應
4. She gave an involuntary smile.
   她不由自主地笑了笑。

in.vol.un.tar.y

adj.（形容詞）
Acting or done without or against one`s will:
非自愿的：無意或非本意而做的：
例句：
an involuntary participant in what turned out to be an argument.
無意中參加了一場爭論的人

Not subject to control of the volition:
不自覺的：不受意志力控制的：
例句：
gave an involuntary start.See Synonyms at spontaneous
不禁嚇了一跳參見 spontaneous

【引伸】
invol.untar.ily
invol.untar.iness
n.（名詞）