You're no fun -- you're ANTITHALIAN

What's the word I'm thinking of? Today, it's...

ANTITHALIAN (adjective) TWITO, page 16

= Opposed to fun

"Mr. Toobad described her as being fully impressed with the truth of his Ahrimanic philosophy, and being altogether as gloomy and antithalian a young lady as Mr. Glowry himself could desire for the future mistress of Nightmare Abbey."
--Thomas Love Peacock, Nightmare Abbey (1818)

"Ahrimanic" refers to Ahriman, which in Zoroastrianism is the name for a devil or Satan. Would you describe Satan as antithalian? It depends on what kind of fun you’re into, I guess.