“Bootstrapping” comes from the term “pulling yourself up by your own bootstraps.” That much you can get from Wikipedia.
In computing, a bootstrap loader is the first piece of code that runs when a machine starts, and is responsible for loading the rest of the operating system. In modern computers it’s stored in ROM, but I recall the bootstrap process on the PDP-11, where you would poke bits via the front-panel switches to load a particular disk segment into memory, and then run it. Needless to say, the bootstrap loader is normally pretty small.
“Bootstrapping” is also used as a term for building a system using itself — or more correctly, a predecessor version. For example, ANTLR version 3 is written using a parser developed in ANTLR version 2.