That phrase appears to be a misheard or slightly altered version of a lyric from the song by Ernie K-Doe , released in 1961. The actual lyric is: "Sent from down below... mother-in-law, mother-in-law."

She never pushes. She just... lingers. If she wants us to go on a family cruise, she doesn’t demand it. She just leaves a brochure on the counter. Then she mentions how much the kids would love the pool. Then she mentions a "great deal" she saw. It’s a slow-burn strategy that eventually makes me say, "Fine, let’s just book the boat!" Why I’ve Stopped Fighting

She never says, “My son’s ex was better.” She doesn't have to. When she mentions how “easy” his childhood was, or how “low maintenance” her husband is, she creates a ghost in the room. You compete with a phantom woman who never existed. To prove you are not difficult, you agree. You bend.

: A humorous, albeit sharp, complaint about the stereotypical overbearing mother-in-law who "puts a wedge" between a husband and wife. Why it sticks in your head