n. The mother of a person's former spouse.
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Scotland's weirdest wedding has been banned at the last minute.

Pat Smith, 44, had been due to marry her toyboy lover George Greenhowe, 22, today.

And George's ex-wife, Pat's daughter Allison, was to be the bridesmaid. But Scots law says a man may not marry his former wife's mother if his ex-wife is still alive.
—Brian McCartney, “Mother out-law,” Daily Record, March 21, 2003
I am very proud of my son-out-laws and when they introduce me as their "mother-out-law" it seems to put paid to the old negative stereotype of the "mother-in-law".
—Joan Kersey, “Judge parents on merit, not marital status” (letter), Sydney Morning Herald (Australia), August 23, 2000
1983 (earliest)
Not long ago, when living together first became a common pattern, people couldn't figure out what to call each other. It was impossible to introduce the man you lived with as a "spouse equivalent." It was harder to refer to the woman your son lived with as his lover, mistress, housemate.

It's equally difficult to describe the peculiar membership of this new lineage. Does your first husband's mother become a mother-out-law?
—Ellen Goodman, “The Very Extended Family,” The Washington Post, August 20, 1983