Condition or fitness. Can be thought of as an analogue to mettle, but regarding functional rather than material qualities. Fettle is how something is made in contrast with mettle being what something is made from.
Found in the 25 January 2014 edition of The Economist. The former military dictator of Pakistan, Perez Musharraf, was described by doctors as being in «pretty good fettle» despite claims of a heart attack for which there was no evidence, and upon which his lawyers claimed he was not healthy enough to be tried for high treason.
Thanks go to The Economist for expanding my vocabulary, a task other publications seem reluctant to take on.