I'm not very familiar with UNIX's history, so take this with a grain of salt, but I believe it originates with the QED text editor. More specifically, with Ken Thompson's rewrite of the QED text editor, when he bequeathed it with an ed-like syntax. (Perhaps more accurate to say "when ed's ancestor began to look like ed".)

In Thompson's QED, much as with ed, the 'g' (aka "global") command's syntax is `g/regular expression/command`. It means "for every line (i.e., globally) matching regular expression, perform command". A common use was `g/re/p` to print matches, thus the now-familiar `grep` binary.

The /etc/glob binary is very similar in feel: `/etc/glob expression command`, which means "for every file matching expression, run command". (Note that the /etc/glob binary didn't return a list of files for the shell to deal with; it did all the command execution on its own.)

I can't pin down exactly when 'g' was added to QED. If we assume Thompson's first rewrite of QED had it, then 1967 might be a good guess. 1970 is the latest possibility, because that's when the only manual I can find[0] is dated. Note that it is for Ritchie's rewrite and mentions an "improved Global command", so it's definitely not the first appearance. There is some more history of QED and ed at [1].

[0]: https://www.bell-labs.com/usr/dmr/www/qedman.html

[1]: https://www.bell-labs.com/usr/dmr/www/qed.html

I don't know the actual reason. However, I suspect its for the same reason why they used "bin" for binary, "usr" for user, "var" for varying, and so forth. There was a culture, back then, of not wasting characters because to do so resulted in a real-life waste of space that could limit what was possible (due to storage size constraints). This extended to the fact that the terminal you typed on was sending characters over a 150 or 300 baud modem. Every chance to save some space while still providing reasonably clear communication was considered desirable.

The question wasn’t about the abbreviation, but about what wildcard expansion has to do with "global".

I knew glob was short for global, but I've never been clear on how the word global applies here. Perhaps it was something like "apply this command globally to all matching files"?

As for the terse abbreviations common Unix commands, recall that much of the early work on Unix was done on very slow teletype devices such as the ASR 33. At 110 baud, brevity is a virtue.