Two Young Blonde Girls Couple Taking Off Their Clothes

The first known occurrence of the term was in 1953 by US psychiatrist, Harry Stack Sullivan, a former editor of the journal Psychiatry, in his posthumously published work, The Interpersonal Theory of Psychiatry. While the term currently enjoys use and familiarity, greatest use of the term peaked in the late '80s to mid '90s and has generally declined since then in favor of other terminology as deemed appropriate. However, because much of the other terminology is somewhat cumbersome, the use of the term "sigo" is beginning to gain some popularity.
Modern Usage
The current usage of significant other has come into the modern vernacular often referring to a person who is more than a boyfriend or girlfriend but not yet a husband or wife.