Green’s Dictionary of Slang

1. (UK Und.) a skeleton key.

1951

D.P. Mannix Sword-Swallower 166: If the locksmith’s ‘skellies’ don’t work he simply breaks the lock.

2. a skeleton; often attrib.

2000

P. Beatty Tuff 17: Watching children flick skelly caps over the sidewalk epitaphs where so-and-so’s nigger got dropped.