SO

The phrase "significant other" was coined in clinical psychiatry rather than in the dating culture that later adopted it. Harry Stack Sullivan introduced "significant other person" in his 1940 paper Conceptions of Modern Psychiatry to describe people who shape an individual's development and self-concept. Wikipedia traces the migration from that technical use into general English in the 1970s, when shifting attitudes toward unmarried cohabitation and same-sex partnerships created demand for a label that did not presume marriage, gender, or living arrangement.

Lifestyle and polyamorous communities adopted the abbreviation SO precisely because of that ambiguity. A poly person can refer to multiple SOs without forcing a hierarchy onto a casual listener; a swinger couple's profile can reference "my SO and I" without disclosing whether that partner is a spouse, a long-term boyfriend, or a nesting partner. Adjacent abbreviations have evolved from the same root: OSO (other significant other) for a partner outside the primary dyad, and NP (nesting partner) for the SO one shares a household with.

Merriam-Webster dates the first known use to 1940 and gives the modern reading as "a person who is important to one's well-being," with particular emphasis on a spouse or comparable partner. The current usage in non-monogamous contexts inverts the singular framing of that dictionary entry without breaking it: SOs are still people who matter, just not necessarily one at a time.