de facto, n. — Green’s Dictionary of Slang

de facto n.

[Lat. de facto, in fact, as opposed to de jure, in law]

(Aus.) one of the two partners in an unmarried but steady relationship.

1991		M.B. ‘Chopper’ Read Chopper From The Inside 95: One of the witnesses was the former de facto of the drug dealer Nick the Greek.
2006		B. Matthews Intractable [ebook] Cox was charged under his real name, Melville Peter Schnitzerling, along with his de facto, Helen Eva Deane.