Fun is an oddly new creation — the word, that is:
“diversion, amusement,” 1727, earlier “a cheat, trick” (c.1700), from verb fun (1680s) “to cheat, hoax,” of uncertain origin, probably a variant of M.E. 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.
“humorous,” 1756, from fun + -y (2). Meaning “strange, odd” is 1806, said to be originally U.S. Southern. The two senses of the word led to the retort question “funny ha-ha or funny peculiar,” which is attested from 1916. Related: Funnier; funniest. Funny farm “mental hospital” is slang from 1962. Funny bone “elbow end of the humerus” is 1826; funnies “newspaper comic strips” is from 1852.