Dictionary.net

Willingly; done with one's consent; negligently. Wolff, §5.

To render an act criminal or tortious it must be voluntary. If a man, therefore, kill another without a will on his part, while engaged in the performance of a lawful act, and having taken proper care to prevent it, he is not guilty of any crime. And if he commit an injury to the person or property of another, he is not liable for damages, unless the act has been voluntary or through negligence, as when a collision takes place between two ships without any fault in either. 2 Dobs. R. 83 3 Hagg. Adm. R. 320, 414.

When the crime or injury happens in the performance of an unlawful act, the party will be considered as having acted voluntarily.

A negligent escape permitted by an officer having the custody of a prisoner will be presumed as voluntary; under a declaration or count charging the escape to have been voluntary, the party will, therefore, be allowed to give a negligent escape in evidence. 1 Saund. 35, n. 1. So Will.