Top definition
Relationship of an individual's mother to his or her partner when no marriage has taken place (for legal or personal reasons).
Doug: Since we live in a state that doesn't recognize gay marriage, I'm not sure what to call your mother. She's not an 'in-law'...
Ed: Call her your mother-out-law. I'm sure she'll like that.
by Mother-out-law November 30, 2011
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