Involuntary (adjective)

1. Not done or acted on by choice or intention; done without one's control or will.
2. Not under the control of the will or conscious mind.
3. Not deliberate or intentional.

Origin:

Late 16th century: from late latin involuntarius, from in- "not" + voluntarius (from voluntas "will").

Examples:

1. The muscle movement was involuntary and beyond his control.
2. The involuntary gasp escaped her lips.
3. The involuntary tremble in his voice betrayed his fear.
4. The involuntary response was a result of the sudden stimulus.
5. The involuntary laughter was inappropriate in the serious situation.