"A fly ball hit to a player during fielding practice in which the batter (often a coach) tosses the ball into the air and hits it as it descends with a long and narrow bat." [Paul Dickson, "The Dickson Baseball Dictionary," 3rd ed., 2009], attested from 1867 (fungoes), baseball slang, of unknown origin; see Dickson's book for a listing of the guesses. Perhaps from a Scottish fung "to pitch, toss, fling;" perhaps from some dialectal fonge "catch," a relic of Old English fon "seize" (see fang), or possibly from the German cognate fangen. Not in OED 2nd ed. (1989). There does not seem to have been a noun phrase (a) fun go in use at the time. It formally resembles the Spanish and Italian words for "fungus."