< classical Latin manūmittere < manū , ablative singular of manus the power of a father or master (lit. ‘hand’: compare manus n.1 2) + mittere to release, send (see mission n.); manū ēmittere is recorded from earlier texts. Compare Law French manumettre to set free (1338 in Middle French), Middle French manumitter (1354), manumiter (15th cent.); also Italian manomettere (end of 13th cent.), Spanish manumitir (early 18th cent. or earlier).
Now chiefly hist.
To release (a person) from slavery, bondage, or servitude; to set free. Also intr.: to obtain one's release from slavery, etc. (OED)
"The Romans, he writes, had no concept of progress: 'The implication is that the order of the universe is static, that social perspectives do not change; they must be the way they are. The 'is' and 'ought to be' of the world are the same.' Thus, a slave might dream of manumission, but hardly of abolition."
- Adam Kirsch, "The empire strikes back: Rome and us", 9 January 2012 The New Yorker, quoting Robert Knapp's Invisible Romans