A mother-in-law is the mother of your husband or wife. My mother-in-law, a mother-in-law joke, visit the mother-in-law.
The plural puts the s in the middle, which catches everybody out. The mother-in-law joke was a staple of British comedy for decades and has largely and mercifully died out.
1phrasemother-in-lawThe mother of your husband or wife.
“My mother-in-law comes to stay with us every Christmas.”
mother-in-law is a relative.
Middle English moder in lawe is mother plus in law, meaning related by canon law rather than by blood.
Middle English
moder in lawe
English, today
mother-in-law
From Middle English moder in lawe; equivalent to mother + -in-law.
Etymology from Wiktionary, CC BY-SA