There's UP and US, and UT—an old name for the first (and last) tone, do, and WE (the funnest pronoun) and WO, which is woe.
“This is going to be the funnest campaign ever,” she told the New York Daily News.
That was one of the funnest parts of my life, driving school buses.
"diversion, amusement," 1727, earlier "a cheat, trick" (c.1700), from verb fun (1680s) "to cheat, hoax," of uncertain origin, probably a variant of Middle English fonnen "befool" (c.1400; see fond).
Stigmatized by Johnson as "a low cant word." Older sense is preserved in phrase to make fun of (1737) and funny money "counterfeit bills" (1938, though this may be more for the sake of the rhyme). See also funny.