Perhaps you were in a chat room or reading a forum, when the abbreviation "SO" came up, along with maybe DH, DS, and/or DD. All of these terms came from the same place - chat rooms in the early 90s - where people had to shorten long words when typing messages to reduce their time online. At the time, most internet connects were made via hard wiring, and the time spent and amount of information shared was limited and/or paid for by the piece.
So. What do the letters SO mean, when talking about relationships? SO refers to a Significant Other, which could mean one's spouse, life partner, or someone they are in a long term, committed relationship with. In turn, DH stands for Darling Husband, DS for Darling Son and DD for Darling Daughter.
Also Known As: partner, husband, wife, significant other
Examples: "I met my SO when we were still in college."