http://projects.csail.mit.edu/gsb/old-archive/gsb-archive/gs... — earliest internet mention (AFAIK), citing Carlin Vieri who coined the term (with this meaning).

https://seths.blog/2005/03/dont_shave_that/ — a good example (and where I first saw it), which actually involves shaving a yak.

Wikipedia, or in this case wiktionary, is your friend: https://en.m.wiktionary.org/wiki/yak_shaving

It’s when your yak’s hair is overgrown and you finally decide to do something about it.

I'm not sure I agree with these definitions in terms of how I've seen it used. To me people tend to use yak shaving to refer to a side distraction project that itself turns into a hairy time consuming affair that doesn't really justify the small benefit you would get from it with regard to the first project; rather than a distraction from a distraction from a distraction.

Something like you need a string function that doesn't seem to exist, and end up rewriting the string library.

You know when you’re trying to fix a bug so you open a debugger but the debugger doesn’t work so you try to edit the config to make it work but the file isn’t where you expect so you try to use find but … eventually you’re trying to shave a yak in order to fix the original bug.