"liberation from slavery, bondage, or restraint," c. 1400, manumissioun, "Christ's redemption of mankind;" early 15c., "freedom from feudal servitude," also an instance of such release, from Old French manumission "freedom, emancipation," and directly from Latin manumissionem (nominative manumissio) "freeing of a slave," noun of action from past-participle stem of manumittere "to set free," from the phrase manu mittere "release from control," from manu, ablative of manus "power of a master," literally "hand" (from PIE root *man- (2) "hand") + mittere "let go, release" (see mission). Specifically in reference to negro slavery in British colonies by 1660s.
The ceremony of the Manumissio by the Vindicta was as follows:—The master brought his slave before the magistratus, and stated the grounds (causa) of the intended manumission. The lictor of the magistratus laid a rod (festuca) on the head of the slave, accompanied with certain formal words, in which he declared that he was a free man ex Jure Quiritium, that is, "vindicavit in libertatem." The master in the meantime held the slave, and after he had pronounced the words "hunc hominem liberum volo," he turned him round (momento turbinis exit Marcus Dama, Persius, Sat. v. 78) and let him go (emisit e manu, or misit manu, Plaut. Capt. ii. 3. 48), whence the general name of the act of manumission. [William Smith, ed., "Dictionary of Greek and Roman Antiquity," 1870]