From mother + -out-law, by analogy to mother-in-law.
mother-out-law (plural mothers-out-law)
- The mother of one's boyfriend or girlfriend.
- The mother of any of one's friends.
- The mother of one's ex-spouse.
2006, Harlyn Aizley, Confessions of the Other Mother: Nonbiological Lesbian Moms Tell All![1] (Social Science), →ISBN, pZgDAQAAQBAJ, page 36:The old Cape Cod commune-istas had caught up enough with my mother out-law and were now asking after notable events in the lives of her offspring.