From the history of Multics, I found that Project MAC was established on July 1, 1963 by MIT for the development of the Multics operating system and later GE (General Electric) and AT&T's Bell Labs joined the project in Aug and Nov. 1964 respectively.
The history of Multics is interesting because the failure of its development led to the development of Unix which is now used in the form of BSD and macOS/iOS and further we have widely used Unix-like operating systems e.g. GNU+Linux.
But from the history of Multics, I couldn't find any information about who had coined the term Multics and when. The only thing related to its naming known to me is "Multiplexed Information and Computing Service."