The informant, KK, is Japanese-American and does not speak Japanese, but can understand it to a degree.
KK: One thing my parents told me a lot was that shaking your legs while sitting leads to you getting poor. It either attracts some figure, or ghostie that lives in your house and sucks up all your dosh.
My parents said it whenever I shook my leg while sitting. I’m drawing a blank on the exact wording but the phrase is “binbo ni nacchauyo!”
I think either the story or the entity is called “binbo yusuri.”
(Japanese script: 貧乏ゆすり)
After I did some digging, I discovered that the first two characters literally mean “poor” and the rest of it is a verb that means “to shake.” It is the official term that refers to the behavioral tic, but it can be considered a proverb because the literal components of the word are not only metaphorical, but also reference a superstitious belief that shaking would cause a child being poor when they become adults.
There is no official consensus on why the Japanese associate leg-shaking with poverty, but there are two most commonly held beliefs why this is the case. The first is that shivering and fidgeting is associated with malnutrition, and the poor are possibly more likely to be afflicted with it. The second, more convincing possibility is that among samurai, fidgeting suggests a lack of self control. Shaking one’s leg thus implies that the person lacks discipline, which in turn suggests a lack of honor.