Came from Unix. A "shell to the kernel" that only exposed the outer layer.

Have a source?

Not really, the internet points to this doc: http://www.multicians.org/shell.html Where Louis Pouzin of Multics claims to have coined it.

Ken Thompson describes it here http://v6shell.org/history/unix/unix.pdf

Looking around, this seems more interesting (implying it is a metaphorical name of a prophet's shell listening): http://unix.stackexchange.com/questions/14934/why-was-the-wo...