1. Any apparently useless activity which, by allowing you to overcome intermediate difficulties, allows you to solve a larger problem.

I was doing a bit of yak shaving this morning, and it looks like it might have paid off.

2. A less useful activity done consciously or subconsciously to procrastinate about a larger but more useful task.

I looked at a reference manual for my car just to answer one question, but I spent the whole afternoon with my nose buried in it, just yak shaving, and got no work done on the car itself.