Mac OS: 7.5.3 Command-Turn On
Yes, that was MacsBug! (IIRC it was a System Extension that had to be installed/enabled -- although it's been quite a number of years...)
I found a command reference for it here: MacsBug Reference.
There's some more info over on Wikipedia.
That was the Debugger window that was built into the Mac's ROM. I never learned what language worked in it.