Not to be gruesome or anything. 😉 I was looking this one up to see when the phrase “skeleton in the closet” came about.
Skeleton itself first arrived in English in about 1570, meaning a mummy, dried-up body, or bone remains. The word came from Latin, but the Latin word had come in turn from Greek, so it’s an ooooollllddddd concept, closely tied to the verb form that meant “to dry up, parch, wither.”
The meaning of “bare outline” followed in about 1600, from which we get “skeleton crew” or “skeleton key.” The phrase that sent me in search of it to begin with was coined right around 1812.