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Decrepit
Weakness or asthenia is a symptom of a number of different conditions. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis.
Wikipedia
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Decrepit (adjective)
Weakened or worn out from age or wear.
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Decrepid (adjective)
obsolete spelling of decrepit17th-20th c.
Wiktionary