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1.
A person in your social circle who only talks to you about one subject and never deviates from said subject.

A.K.A. SSF

NOTE: The single subject does not include talking about one's self.
It's like when you meet someone and talk to them about bittorrenting and they do nothing more than talk to you about bittorrenting regardless of how you try to move the subject to another conversation, hence they are a single-subject friend.
by flippedlid July 13, 2010