son-out-law

From son +‎ -out-law, by analogy to son-in-law.

son-out-law (plural sons-out-law)

1. A live-in boyfriend of one's child.
   • 2013, Lucien Nzeyimana, Burning the Last Straw, page 339:

     “You also treated Haydar like your own son,” Didace observed. “He was my son-out-law, remember,” Henock prompted.

2. The ex-husband of one's child.
   • 2014, Christopher W. Moore, The Mediation Process, page 540:

     The parents decided to give their daughter and “son-out-law” the cold shoulder.