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A person you spend a lot of time with, talk candidly with and do date like stuff with and while you could have a physical and/or emotional relationship, you don't.

S.O. is an abbreviation for significant other.
Even though they are of the opposite sex, they spend a lot of time together and have inside jokes, they aren't boyfriend and girlfriend, they are just faux-S.O.
by JroDC March 01, 2008