[zem]

the version I learnt the phrase from had the end of the chain as

- oh we should paint it

- we need a paintbrush

- I hear yak hair makes the best paintbrushes

- here I am, shaving a yak

made more sense than the examples given in the op

[rzzzt]

Internet Search Thing now tells me sable (for soft brushes) and hog hair (for wire-like brushes) is the best hair for this purpose. Oxen occasionally show up. Yaks nowhere to be found.

Coincidentally, this is also the first time I'm hearing about sable.

[clscott]

So yak shaving comes after bike shedding?