The inability to laugh, relax, take it easy, or find jokes funny. People with sense of humor cancer are typically perfectionists who never laugh. They can also appear as radical conservative assholes like Ann Coulter, or liberal hippie douchebags that have to defend everything. And if they smile at anything, it's the most vapid form of obvious humor. They take everything absolutely seriously and are usually an absolute buzzkill.
Sense of humor cancer can metastasize so badly that the victim may hate and be jealous of others who actually have a sense of humor.
"My sister lives in California, protests in demonstrations, hates all men, and never laughs at anything. I think she has sense of humor cancer."