a.

1. 非自願的；非出本意的
   He gave an involuntary smile.

   他勉強笑了笑。

2. 非故意的；無意的；偶然的
   an involuntary injury

   無意的傷害

3. 不受意志控制的；不由自主的
   Sneezing is involuntary.

   打噴嚏是不由自主的。

adj.

1. done without conscious control.
   ▸(especially of muscles or nerves) concerned in bodily processes that are not under the control of the will.
2. done against someone's will.