Bare metal comes from mainframe days. The largest systems like IBM 360-67 and others were called "Big Iron" - see https://www.ibm.com/ibm/history/ibm100/us/en/icons/system360... for NASA's moonshot-era Big Iron. You could run programs on OS/360 or you could, if daring, run code directly on the machine, on the "Bare Metal". It's quite common to run 1-card programs on the Computer History Museum's IBM 1401 --- which was not really considered 'Big Iron' at the time, although today we call it a 'Mainframe' https://computerhistory.org/exhibits/ibm1401/
Interesting. But why were they called big iron? It doesn't seem to be in IBMs marketing material (at least I couldn't find it) and the wikipedia reference points to a 2005 vulture article where they call it the big iron[1]
