There's another wordplay in the name.
Namely, that "SB" stands for "Sanely Bootstrappable" - something that the CMU Common Lisp was unable to do
It is my understanding that easing bootstrap was the original motivation of the fork of CMU CL. I'm a bit confused about that, since CMU CL supports compilation to byte-code (which should make a port to a new architecture comparatively easy), a feature which was dropped in SBCL.
That's more of a backronym.
In my first job, grumpy Martin had as one of his many indispensable tasks, to build cmucl. (Itasoftware, one of the main lisp shops of the age)