Nobody seems to know for sure. A practice fly ball hit to a fielder with a specially designed bat. Below, you will find two particular offerings and a sprinkling of assorted opinions and details.
All credit is given to sources when known.
He was probably the world's greatest fan and the most knowledgeable individual then or now, on baseball and some other things, that I've ever known. Homeruns were the rule because of the incredible batspeed and compression energy the fungo bat could generate off the rubber-wound balls. Often times, the opposite field was eliminated from play and five-man teams were it. I'll keep searching for a verifiable source on this subject. There is reference to the warm-up for cricket matches involving 'fun goes,' i.e. practice strokes before the game began in earnest.