Is it ensure, insure, or assure?
"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.