This definition appears very rarely
Samples in periodicals archive:
Is it ever okay to snoop through your significant other's private papers, email, phone, journals etc.?
I keep having dreams about my ex-significant other (we have been broken up for six months). The dreams aren't about getting back with him; just about talking to him.
The Ex significant other joke thread. General NON-Automotive Conversation
(Ex) significant other is used in Emoticon. The word (x)SO is used in Emoticon meaning (Ex) significant other
Dear Miss Manners: My significant other told me she would like to continue the friendship but abandon the rest of the relationship. Although that took some getting...