“A hair's breadth”
Hello! I’m curious, is this idiom still commonly used in modern speech? Do you hear it often?

Question

Answer

It's sometimes heard in poetic usage, for example on TV. But I'd be very surprised if someone said it to me in person. If they did, I would ask why they're speaking poetry.

Answer

No, not often, but everybody has heard it and recognizes its meaning.  There are many such expressions which, while not heard in everyday speech, are instantaneously recognizable to almost anyone:

"when pigs fly"
"at the drop of a hat"
"root and branch"

and thousands more.

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