Why was the word "shell" used to descibe a command-line interface?

The analogy is with a nut: outside is the shell, inside is the kernel.

From http://en.wikipedia.org/wiki/Thompson_shell#History

"The name "shell" for a command line interpreter and the concept of making the shell a user program outside of the operating system kernel were introduced in Unix's precursor Multics."

Further down the wikipedia rabbit hole finds this: "Louis Pouzin, introduced the term shell for the command language used in Multics"

I have not read it in full, but his writing here may give you the answer: http://www.multicians.org/shell.html

EDIT: indeed it is not explained. He merely "coined" the term. Not the funny story you were hoping for, was it?

Maybe because it is the surface, shielding the inner kernel from the user? So the kernel would be the pearl inside the shell.