Bare metal has meant "whithout a supervisor" to operations for more than a decade. It has also meant "without an emulator" to the emulation community for a really long time.
I imagine it also has some meaning for the music community.
I suspect it comes from the automotive paint industry. Sanding down to the "bare metal" for the best finish...where the primer and paint are as close to the substrate as they can be.
I imagine it also has some meaning for the music community.