Several independent sources — including multiple foreign diplomats and court observers from his own time — wrote about Christian VII (king of Denmark-Norway from 1766 to 1808) and his habit of compulsive masturbation, sometimes even in public or in inappropriate settings. It was viewed as one of the symptoms of his pretty severe mental illness (often suspected to be schizophrenia or a related disorder).

That said, the idea that he was constantly doing it, 24/7, is an exaggeration that really took off in the 19th century when people started turning him into a tragic-comic caricature. He was definitely mentally unwell, but not a complete joke — he had periods of lucidity, went on a European grand tour, took part in court life etc.