If you had been around that generation of systems and toggled in a 30 byte program in hex or octal that loaded a paper tape or card deck that then loaded the actual OS, bootstrap seemed a reasonable term.
Bootstrap actually makes some sense when you're toggling in a loader on a machine that has a bunch of switches, it certainly feels like you're trying to lift yourself up by your laces rather than that you're loading an initial program.
Oh, and one bit wrong and you can start all over again... the scary thing is that once you've done it often enough you start to remember the sequences the same way you remember how to play a piece on the piano, in your muscle memory.
