Granny Weatherwax - what's the joke?

I've been reading pratchett forever and a day. I was quite happily accepting that esme weatherwax was a simple name for a character.

But.. then i realised about vetinari and gytha ogg and names aren't just names, but still i accepted Weatherwax.

Then a few years ago i saw qa passing reference to her name being a joke. But none explained what. And it has been bugging me for aaaaages.

Will someone please explain to me why granny weatherwax is an amusing name?

Thanks awfully!