I know I'm super late on this, but remember a couple of months back when we were talking about Phil and his so-called "Sylvanio thumb"? A few days ago the topic popped back into my mind, and I realized that "Sylvanio" sounds an awful lot like "Sylvania", a company that makes TV remotes.
Sounds to me like "Sylvania thumb" would be a condition that his dad jokingly made up to refer to his wife's excessive channel surfing, to the point that her thumb gets swollen and sore.
Naturally, young Philliam overheard this, misinterpreted it, and in his MUHCHUR ADOLT years, decided to give it an awthenic Itaaahlyin twist to make it sound more exotic.