Likely a fictitious disease: An unhealthy (presumably fatal) extremely large persistent erection.
Coming from the definition of:
- Chronic ("persisting for a long time or constantly recurring.")
- Morbid ("the nature of or indicative of disease.")
- Tumescence ("swollen or becoming swollen, especially as a response to sexual arousal.")
Mentioned as the cause of death of Henry Flanken who possessed a genie. As Mulder describes it, "Schwing. On April 4, 1978, he was admitted to Gateway Memorial Hospital with an extreme priapic condition. Apparently, he was quite the specimen. They had to raise the door frame in order to wheel him into his hospital room."