It's often used in phrases like:
* "In good fettle": Meaning someone is healthy, happy, and energetic.
* "In poor fettle": Meaning someone is unwell, tired, or in low spirits.
* "To put someone in good fettle": Meaning to make someone feel happy and healthy.
Example:
* "After a good night's sleep, I woke up feeling in excellent fettle."
* "The old car is in pretty poor fettle, it needs a lot of repairs."
It's a slightly old-fashioned word, but still commonly used in British English.