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1HB an involuntary movement, sound, reaction etc is one that you make suddenly and without intending to because you cannot control yourself : 
When Willie tapped on a window, Miguel gave an involuntary jump.
When Willie tapped on a window, Miguel gave an involuntary jump.2 happening to you although you do not want it to : involuntary part-time workers
involuntary part-time workers —involuntarily adverb
