bss = "Block Started by Symbol"
Dennis Ritchie says:
Actually the acronym (in the sense we took it up; it may
have other credible etymologies) is "Block Started by
Symbol." It was a pseudo-op in FAP (Fortran Assembly [-er?]
Program), an assembler for the IBM 704-709-7090-7094
machines. It defined its label and set aside space for a
given number of words. There was another pseudo-op, BES,
"Block Ended by Symbol" that did the same except that the
label was defined by the last assigned word + 1. (On these
machines Fortran arrays were stored backwards in storage
and were 1-origin.)
The usage is reasonably appropriate, because just as with
standard Unix loaders, the space assigned didn’t have to be
punched literally into the object deck but was represented
by a count somewhere.