Well, the "where to wear it" is "don't bother." It doesn't do anything. Logic would indicate that it's a fairly new piece of folk belief, however, since the safety pin is pretty new, historically speaking.

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Gillian

"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"

"You can't erase icing."

"I can't believe it doesn't work! I found it on the internet, man!"