SO
Significant other
Example

Some SOs can be a bit ... overbearing
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Last Updated: January 7, 2022
Strikeouts
An abbreviation used in baseball statistics to count the number of strikeouts by a pitcher or batter.
The SO abbreviation is commonly used to measure a pitcher's efficiency along with his ERA, W, L, and CG. It also appears in a batter's statistics to measure how well he is hitting. The SO abbreviation can be used interchangeably with K to keep track of strikeouts.
Example
Related Slang
Usage
Last Updated: October 1, 2014
Those lucky enough to be in a romantic relationship use the acronym SO to refer to their "significant other." A person's significant other is their romantic partner. Notably, the term SO is gender-neutral, so it can be used to refer to a person's bf, gf, husband, wife, or other type of partner.
As you might expect, you're most likely to encounter the SO acronym when talking with someone who is part of a couple. Some couples refer to each other as SOs simply because they prefer the term to its alternatives, while others do so to avoid "defining" their relationship too early.