사회주의 : 社會主義 (society + ideology)
자본주의 = 자본capital) + 주의(ideology)
Didn't we already have this in 채식주의(자) for vegetarianism(ist -> vegetarian) early on?
The word looks like it is possessive. Is it?
In this case 의 comes from the Middle Chinese 義, literally meaning something like "justice". The combination 주의 means something like "-ism" (in this case it's society + -ism = socialism).
Excellent question and excellent answer.