What does “be an open secret” mean?

if sth is an open secret, many people know about it, although it is supposed to be a secret

How is “be an open secret” used?

In Maria Bamford: Old Baby (2017), one line is: “That's an open secret.”

The clip at the top of this page is from Maria Bamford: Old Baby (2017).

That's an open secret.

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