[my own comments marked by square brackets]
Informant: “My mom would tell me if I–like the nervous habit of shaking your leg, or like shaking your foot, whenever you’re not doing anything, gives you financial bad luck.”
[This is a Korean thing right?]
“Yeah it’s a Korean thing. But I mean there’s like no logical reasoning behind it, which I think makes it really Korean *laughs* But like yeah, she would just tell me ‘Don’t shake your leg’ and it’s something that I do now to like other people–not to tell them it’s bad luck but it became like this process of just stopping people from shaking their legs.”
This type of superstition arbitrarily equates a certain habit with an undesirable outcome in order to make that habit seem inherently undesirable and thereby prevent people from doing it. It also shows what Koreans consider to be a high stake concern–financial security–by designating it as the negative incentive, of all things that could have been chosen. Idle shaking of the leg is a restless habit that suggests anxiety, restlessness, and impatience, which already makes it unpleasant. Perhaps the superstition is based on rash financial decisions that were made in moments of restless impatience, and then retroactively connected to the habit. Another possibility is that people just don’t like to be around leg shaking for intuitive reasons–its nervous quality can make other people feel nervous just by looking at it–which therefore makes it somewhat easy to adopt the superstition after it’s heard.