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Wiktionary
unfun

a. Not fun. n. Lack or absence of fun; funlessness.

Wikipedia
Unfun

Unfun is the first album by the band Jawbreaker. It was first released on Shredder Records in 1990. The album was remastered by Jawbreaker drummer Adam Pfahler in 2008, and was reissued March 30, 2010, adding the 7" mix of "Busy".

Usage examples of "unfun".

Now, after the pressures of their decidedly unfun cruise, she was ready to do things his way.

We would forget all those stuffy, weird shadow lurkers and hammer pounders and generally unfun, gloomy-gus guy gods.