she asks if we can confirm that this is a familiar expression.

Remembered this site from doing my first search, Keiva. This quote should confirm it as an old, common saying for her. It's also in the body of the article. And make sure you direct her to the author's credentials at the bottom of the page, that'll confirm it as a solid source for her. Hope this helps!

[edit:] Looking over Mr. Gardner's credits it seems he's also something of a folklorist. He has his e-mail posted, so maybe he has the answer that's been stumping us all about the origin of the phrase. Have her try him and see.