To be in fine fettle

It dates back long before the age of steam, the first quotation in the OED being from the early 15th century. The principal meaning of the verb (there are others) means to 'gird up', or to 'make ready', and the noun, that the subject is ready and in fine order.

I believe it was extensively used with preparing horses and that is from where the modern usage derives; which is spoken with mild irony and jest. 'She's in fine fettle'.

Pronunciation: /ˈfɛt(ə)l/ Forms: ME–15 fettel, ME–16 fetle, (ME fettil, fetyl), ME–15 fetel(e, 18 dial. fottle, ME– fettle.(Show Less) Etymology: Possibly < Old English fętel, fettle n.1; the primary sense would then be ‘to gird up’.(Show Less)

a. trans. To make ready, put in order, arrange. Now only dial. to put to rights, ‘tidy up’, scour; also, to groom (a horse), attend to (cattle).

a1400–50 Alexander 626 And faste by his enfourme was fettild his place.

c1400 (▸?c1390) Sir Gawain & Green Knight (1940) l. 656 Now alle þese fyue syþeȝ, forsoþe, were fetled on þis knyȝt.

(Oxford English Dictionary)

To fettle is to adjust a piece of machinery to obtain optimum performance.

Frequently used in the age of steam to refer to steam engines, agricultural equipment, cotton mills etc. Such machines usually contain belts of some sort, which need constant adjustment or fettling.

Machinery running well is said to be in fine fettle, so by extension, if you are in fine fettle you are well adjusted and in good health.