You always hold a string in your mouth when someone is cutting or pinning something on you, and that way they won’t cut you by accident, or stick you with the pin. I’m not sure how or why, but my grandmother was a seamstress and her mother was a seamstress, and you can bet nobody ever pinned or cut anything on me, when I was trying it on, you know, without a string in my mouth. My mother and my grandmother, they just wouldn’t do it. I don’t know why, it’s just how it was.
Informant does indeed come from a family of seamstress. The interviewer was unable to verify other sources for this superstition, but it is clearly a sort of contagious magic: that if the person holds a string in their mouth, the wholeness of that string will somehow keep them whole. It probably also serves a more practical purpose–reminding the person to stay still and quiet while the seamstress works on them.