1
general::
A shameful secret, as in Both her parents were alcoholics; that was the skeleton in her closet. This metaphoric term alludes to a murder victim long concealed in a closet, possibly based on some true incident that is now forgotten. [Early 1800s]
American Heritage Idioms
2
general::
Phrase(s): skeleton(s) in the closet
a hidden and shocking secret. • You can ask anyone about how reliable I am. I don’t mind. I don’t have any skeletons in the closet. • My uncle was in jail for a day once. That’s our family’s skeleton in the closet.
McGrawhill's American Idioms And Phrasal Verbs