What the hey-hoo is a bootstrap?
Have you ever seen that loop at the top of the rear of the boot?
That is a bootstrap. It is to help you pull on your boots.
I found a fairly good pic of one:
I have a pair kangaroo leather cowboy boots that I rarely wear. The last time I put them on, I asked my gf to help me. She told me to pull them on by my own bootstraps (haha) and I did. They have a loop on each side instead of the back.
I don’t know if it’s universal, but in my experience lace up boots have a single strap in the rear while tall slip-on boots (like cowboy boots etc.) will have two loops inside, one on each side. Presumably, to give one more leverage to pull oneself up by.
Hey. If you pull yourself up, by bootstraps, wouldn’t you fall over? Twice(2 boots)?
Seems counterintuitive to success.
Probably so. Physics gets in the way of so many good aphorisms. 
From a Douglas Hofstadter column: “This analogy is like pulling yourself up by your bootstraps.”
Lean down, grab your bootstraps, and pull yourself into the air. Easy.
Hence bootstrapping any system that in some sense already needs to be running in order to run.
Examples being:
The power grid, where generators need electrical power to energise their field coils before they can generate power. The problem usually being know as a blackstart.
A computer. Where the operating system that runs the computer, including the code needed to access devices - such as a hard disk - resides on a hard disk. So how do you access the operating system on the disk to start it all off? Which is known as bootstrapping a computer, or nowadays simply booting.
Interestingly, the meaning of the phrase shifted over the years-- it was originally meant to be sarcastic, meaning something impossible. Then it eventually came to be an encouragement to take charge and lift oneself out of dire or poor circumstances:
Not merely sarcastic, but mocking.
The mutation into something desirable and admirable is quite puzzling.
There are other similar examples of words, maybe not so much entire phrases, that changed meaning and in some cases took on the opposite meaning over time. Calling somebody ‘sophisticated’ used to be an insult, meaning something like deceptively cunning and manipulative. It gradually took on a much more positive connotation.
Why, I’d say there’s literally a billion similar examples of words whose meaning has changed over time!
If you pull hard enough you should be able to do a flip…
The idea predates Hofstadter. In 1941, Heinlein wrote “By His Bootstraps.”
One that’s gone the other way is “condescending.” At one time it was a fairly neutral, or even positive, term, meaning that someone could put aside the privileges of their rank and treat their social inferiors with equality. In Pride and Prejudice, Jane Austen described a character as “all affability and condescension,” and meant it as a compliment.
To “boot up” a computer likely comes from this idea. A computer needs an operating system to enable it to load and run applications, but an operating system is itself just a program, so how does it load the operating system in the first place? It needs to pull itself up by its bootstraps.
When I was a kid (and the expression still had the “impossible” connotation), I always pictured someone with extremely long bootstraps, going up and over some sort of scaffolding or other framework, and then back down, so that the person wearing said boots could pull down on the straps, and so lift themself (with a factor of 2 mechanical advantage, even). I never understood why that was considered so impossible.
‘Man up’ kinda bothers me too.
If you ride English not only do your boots have bootstraps, but it’s very likely you own boot hooks and maybe even a boot jack. Those tall boots are a PITA.
But, I so want a crop. It has a loop.
Bootstrapping is itself a concept. in computing, but it refers to writing a compiler that can compile itself on that platform. Often it involves making a simpler compiler first.