The phrase "metal skeleton" refers to a structure made of metal that is shaped like a skeleton. It is meant to highlight the strong and rigid nature of the metal material. Full definition
A few weeks ago I found some awesome metal skeleton keys at my local thrift store. (simplydarrling.com)
-- Hreinn Fridfinnsson's fourth house of the house project since 1974, an ethereal shining metal skeleton of a house situated in a sylvan clearing outside the city center. (artnews.com)
Batteries in each leg power Kengoro through about 20 minutes of exercise at a time, and water seeping from inside Kengoro's metal skeleton like sweat keeps the motors of the artificial muscles cool while the bot works out. (sciencenews.org)