The phrase "in good fettle" is often used synonymously with the words healthy, fit, and robust. Other synonyms for this phrase include "in good health," "hale and hearty," "sound," and "well-conditioned." These words all describe a state of physical or mental health where one is in top form. Other possible synonyms may include "vital," "energetic," or "spry," indicating a sense of increased energy and an ability to perform at an elevated level. These words can be used in a variety of contexts, from describing a person's physical fitness to describing the condition of a machine or piece of equipment. Regardless of the specific scenario, the phrase "in good fettle" suggests a sense of strength and vitality that is both admirable and desirable.