'Skeleton key' definitions:

Definition of 'skeleton key'

noun

A passkey with much of the bit filed away so that it can open different locks

From: GCIDE

Skeleton \Skel"e*ton\, a. Consisting of, or resembling, a skeleton; consisting merely of the framework or outlines; having only certain leading features of anything; as, a skeleton sermon; a skeleton crystal. [1913 Webster]
Skeleton bill, a bill or draft made out in blank as to the amount or payee, but signed by the acceptor. [Eng.]
Skeleton key, a key with nearly the whole substance of the web filed away, to adapt it to avoid the wards of a lock; a master key; -- used for opening locks to which it has not been especially fitted.
Skeleton leaf, a leaf from which the pulpy part has been removed by chemical means, the fibrous part alone remaining.
Skeleton proof, a proof of a print or engraving, with the inscription outlined in hair strokes only, such proofs being taken before the engraving is finished.
Skeleton regiment, a regiment which has its complement of officers, but in which there are few enlisted men.
Skeleton shrimp (Zool.), a small crustacean of the genus Caprella. See Illust. under Laemodipoda. [1913 Webster]