[mass noun] A yellow crystalline compound which is present in many lichens and is used as an antibiotic.
- ‘Bitter orange, aristolochic acid and usnic acid are all used for weight loss.’
- ‘I don't know anyone else who's working on the toxicity of usnic acid besides me.’
- ‘Although several synthetic compounds inhibit this key pigment enzyme, the scientists found that usnic acid was over 10 times more effective than others tested.’
Mid 19th century: usnic from medieval Latin usnea (from Arabic ushnah moss) + -ic.