Why key is called skeleton key? [closed]

One old type of lock was the "warded lock". Inside the lock were a series of protrusions called "wards" which the key would have to pass to operate the locking mechanism and open the lock. Each lock would have different patterns of wards and the keys would have corresponding cutouts. If you used the wrong key then it would get blocked by the wards in the lock.

The fatal flaw in this design was that you could take a key blank and cut away most of the material so it would pass the wards in all the locks and hence open all of them. Such a key was known as a "skeleton key", presumablly because what was left was the bare skeleton of a key with just enough material to hold the key together and activate the lock mechanism.

Lever locks were not susceptible to generic skeleton keys. However master key systems based on lever locks generally used the same levers in every lock and then used wards to control which keys would open which lock. So someone could take a regular key from the system and turn it into a master key by removing material until only a skeleton of the key was left.

I could be wrong but I think that the title is supposed to have a dual-meaning referring both to the regular meaning of the term "skeleton key" but also to the fact that the key is unlocking the "skeletons in the closet"

It's referred to as a skeleton key because that's what a master key is known as. From here:

The term derives from the fact that the key has been reduced to its essential parts.

Generally, a skeleton key is a type of master key in which the serrated edge has been filed down so that it can open numerous locks, most commonly the warded lock.

In terms of movie, Violet gives Caroline (Kate Hudson) a skeleton key, since she’s going to be a live-in hospice caretaker for Ben. The key, opens every door in giant 30+ room mansion…except for one, the one in the attic lies a secret room, supposedly locked forever.