Others not so much, but in sbcl-0.7.0 I put some effort into
making them more consistent.
ARG argument
+ DX dynamic-extent
FUN function
GC garbage collect(ion)
N new: number, as in e.g. N-PASSES or N-WORD-BITS
old: conventional prefix for temporary variables used to
implement evaluate-only-once semantics in macros
+ NLE non-local entry
NLX non-local exit (in compiler IR2)
SB storage base (in compiler IR2)
SC storage class (in compiler IR2)