Short for fibromyalgia or a prefix meaning fiber or fibrous; used informally to refer to fibrous tissue.
From Latin fibra (fiber). As a standalone noun, 'fibro' is modern slang (20th century) for fibromyalgia; as a prefix, it's been used since the 1800s.
People with 'fibro' (fibromyalgia) experience widespread muscle pain, and interestingly, scientists still don't fully understand why—it's one of modern medicine's puzzles.
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