Yak shaving is a term invented by Carlin Vieri at the MIT AI Lab. Yak shaving is what you are doing when you're doing some
stupid, fiddly little task that bears no obvious relationship to what
you're supposed to be working on, but yet a chain of twelve causal
relations links what you're doing to the original meta-task.
Here's an example:
"I was working on my thesis and realized I needed a reference. I'd
seen a post on comp.arch recently that cited a paper, so I fired up
gnus. While I was searching the for the post, I came across another
post whose MIME encoding screwed up my ancient version of gnus, so I
stopped and downloaded the latest version of gnus.
"Unfortunately, the new version of gnus didn't work with emacs 18, so
I downloaded and built emacs 20. Of course, then I had to install
updated versions of a half-dozen other packages to keep other users
from hurting me. When I finally tried to use the new gnus, it kept
crapping out on my old configuration. And that's why I'm deep in the
gnus info pages and my .emacs file -- and yet it's all part of working
on my thesis."
The above is taken fairly directly from a post to the AI Lab's mailing list by Jeremy Brown, a tenured graduate student.