E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To set free; properly to send from ones hand (e manu mittere). One of the Roman ways of freeing a slave was to take him before the chief magistrate and say, I wish this man to be free. The lictor or master then turned the slave round in a circle, struck him with a rod across the cheek, and let him go.