Way back when, (not really that long ago) US Air bought a few smaller airlines.....Piedmont, Allegheny, and PSA a.k.a. Pacific Southwest Airlines. Since they now owned the rights to those airlines names, they gave them to thier wholly-owneds. But the present day PSA used to be Jetstream Intl. Airlines, so they are not related to each other in any way. So to answer your question, PSA really does not stand for anything.
I think