A literary or artistic assistant, in particular one who takes dictation or copies manuscripts.
Who the author was we do not know with certainty, but... a reference at the end of I Peter... makes him the companion and amanuensis of Peter. (H L Mencken, Treatise on the Gods)
May I introduce my niece, Evelyn Dixon, as an honest and competent secretary. For the past five years, Miss Dixon has acted as amanuensis to Sir Wilfred Hadley, my late father. (Judith B Glad, The Anonymous Amanuensis)
We are allowed to drink it, mercifully – Pratchett’s assistant and amanuensis, Rob, a large and friendly man in late youth, had recommended it the moment the pair of them walked in the door – and the knobbly bits can remain in situ. (Tom Chivers, Terry Pratchett interview: a fantasy writer facing reality)
Early 17th century: Latin, from (servus) a manu '(slave) at hand(writing), secretary' + -ensis 'belonging to'. (Oxford Dictionaries)
In Latin, the phrase servus a manu translates loosely as "slave with secretarial duties." (The noun manu, meaning "hand," gave us words such as manuscript, originally meaning a document written or typed by hand.) In the 17th century the second part of this phrase was borrowed into English to create amanuensis, a word for a person who is employed (willingly) to do the important but sometimes menial work of transcribing the words of another. (Merriam-Webster)