The "S" stands for Satan.
From:
http://sites.google.com/site/earthdeception/
Fake Sionist Universe Exposed!
( Sion = Satan's Statist/Political Zion,
which is NOT the same as
So from now on don't call the bad guys "Zionists" but instead call them "Sionists" or "Political Zionists".
From:

Sion is a synonym for Zion, as in the French Sioniste. Also see here.
to them the land is Zion, so that's what they call themselves, & what I'll continue to call them. They get enough ambiguity already.