An open secret is a concept or idea that is "officially" secret or restricted in knowledge, but is actually widely known; or refers to something which is widely known to be true, but which none of the people most intimately concerned are willing to categorically acknowledge in public.
Famous quotes containing the words open and/or secret:
“They never open their mouths without subtracting from the sum of human knowledge.”
—Thomas Brackett Reed (1839–1902)
“Nothing is covered up that will not be uncovered, and nothing secret that will not become known.”
—Bible: New Testament, Luke 12:2.