The Latin word for "fun" is ludus. Alternatively, one could also use iocus, at least according to Cassell's Latin & English dictionary. Ordinarily, I wouldn't mention it, but it came up in conversation tonight. Or rather, someone at an SCA meeting stated authoritatively that there was no Latin word for "fun", one of those weird little factoids one frequently hears in the SCA (such as "the color pink didn't exist during the SCA period", or "only servants wore blue", or "one time an SCA King sold homemade porn to finance his reign".)
Oh, wait, the last one was actually true.