I always thought it was in reference to which hand a person uses to masturbate. Learn something everyday. 

 LMAO 

 I always heard it came from the Roman senate.  People with one set of views were on the left side of the room and the others were on the right. 

 Interesting. Do you know what characteristics might have been associated with each side?  

 I think it was people who liked the status quo vs people who wanted to change things.  It's been a long time since I studied history.