Noun
One who resides on the south side of a municipality or other location.
One who resides on the south side of Chicago, USA.
(baseball) A player or fan of the Chicago White Sox baseball team, which plays its home games on Chicago's south side.
I'm no baby bear fan; I'm a Southsider.
(soccer) A fan of the Vancouver Whitecaps football team
Source: en.wiktionary.org