The Brief: SO is an abbreviation for significant other.
The term SO, or s/o, stands for significant other and is commonly used on Reddit as a gender-neutral way to describe one’s partner.
For example, this Redditor made a Zelda themed birthday cake for their significant other:
accepting s/o (significant other) applications ! pic.twitter.com/YVwhwTfQuy
— jooheon’s big spoon (@heonyschou) October 6, 2019