fettle \FET-l\, noun:
A state or condition of fitness or order; state of mind; spirits — often used in the phrase “in fine fettle.”

dictionary.com

Now although I have heard the phrase “in fine fettle”, around here, to fettle is a term used to describe having a damned good clean of something, as in “That floor’s filthy; I’m going to give it a good fettle!”

It’s not a phrase I’d say I’ve heard often, or used an awful lot these days, but according to the Steel Founders Society of America, it is,

British term meaning the process of removing all runners and risers and cleaning off adhering sand from the casting. Also refers to the removal of slag from the inside of the cupola and in Britain to repair the bed of an open hearth.

SFSA

That kind of makes sense actually, because it’s not a word I’ve heard commonly in other parts of the country, and here in Derbyshire we’re not a million miles away from the borders of South Yorkshire, and of course areas of Yorkshire, and in particular Sheffield, which is only a few miles up the road, are, or formerly were major steel producing centres, and my mum, who’s the one person I know who regularly uses the word, grew up in Yorkshire (though she doesn’t like to admit it ). I also remember my granny, when I was much younger, and in the days when they had an open coal fire talking about “fettling the grate.”

So there you go; a local word that comes from the area’s former heavy industry. Obviously no-one told those nice people who wrote the dictionary that though!