It's one of those cases of derived derived meaning. As a simplfied definition, skeleton keys were for a kind of lock that supposedly the key of resembled a skeleton. However, often any skeleton key could open any type of warded lock (a lock that relies on obstrustions that prevent wrong keys from turning) due to bad design of the lock, or good design of the key. Thus, it became incorrectly known that skeleton keys open any warded type lock. Then, it bacame known that skeleton keys open any lock, which is 100% incorrect. There is no key in existance than can do that.
Now that's out of the way, where is that skeleton key? We need to open all the locks. *wacko*