A friend of mine once said that W-body means widebody...
I had to tell him that GM chassis designations had no real meaning and were just there as an identifier for the chassis.
There is nothing more to the the designation of W than W itself. Unless you know of the prototype designation GM10. But even that does not suggest more than the preproduction test mules of the mid '80's.
That's a common misconception, that the W stands for "Widebody". Truth be told, the 1st generation W's were actually narrowbodied. The 2nd gens were the ones that grew a bit wider. By the 3rd gen, the cars were considered "full size" rather than mid size. For what it's worth, the W body is part of the RWD A body (later G) lineage. The model names of our cars stem from cars on the A body platform in the late 60's and early 70's, hence its roots as a mid size car since the W was the chassis that successfully "killed" the RWD mid size program. The FWD A body tried but failed.