Sozi ist die Kurzform für einen Anhänger des Sozialismus oder der Sozialdemokratie.
translates roughly to
"sozi is the short form for a follower of socialism or the socialdemocracy"
Analog zur Kurzform Sozi für Sozialist existiert auch die Kurzform Nazi für Nationalsozialist.
"analogue to the short form sozi, for socialist, exists also the short form nazi, for nationalsozialist."
now, that's from the german wikipedia, i suppose that's not a good enough source, so let me look it up somewhere else.
oh, look, it's from a german liberal website (liberal in the classical sense of the word, not the american, so i guess these guys have their socialist abbreviations right, no?)
Sozi,
der; -s, -s
verharmlosendes, familiarisierendes Diminutivum für Standard-, Neo- und Krypto-Sozialisten. Sozialisten setzen das Diminutivum, gerne im Eigengebrauch ein. Nach DUDEN allerdings abwertende Kurzform.
now, my german is far from perfect, but a roguh translation would be:
"belittling, familiarising, diminutive for the standard-, neo- and krypto-socialist. the socialist is likely to use the diminutive themselves (i'm not sure aobut this part). this is a very derogatory short form(i'm not sure about this either)"