Used in 针灸 ('acupuncture and moxibustion') and 艾灸 ('moxa-stick therapy'). Moxibustion burns dried mugwort (艾) near acupoints to warm the body; do not confuse 灸 with the look-alike 炙 ('to roast').
The fire radical at the bottom gives the meaning: moxibustion is a heat therapy that burns moxa near the skin, so the word is built on the fire radical.
On top it supplies the sound: 久 jiǔ gives the reading directly. The sense of 'sustained, lasting' also faintly suits holding heat against a point for a prolonged time.
