Those wacky college kids.
In the 1870s, the Frisbie Pie Company of Bridgeport, Connecticut delivered pies to Yale students. Some mastermind in the 1940s got the idea it would be fun to toss the empty pie tins around. It became a popular diversion.
Then a man named Walter Fredrick Morrison, a UFO fan, developed a metal toy disk, later produced in plastic, that mimicked the flight of the UFOs in the movies (I don't know if there were any patterned off real UFOs. It probably didn't test market as well.) With the help of Wham-O, these hit the shelves as Flyin' Saucers. While on an East Coast promotional tour, the president of Wham-O discovered Harvard and Yale students tossing around pie tins they called "Frisbies". Unaware of the pie company, Wham-O's president Richard Knerr decided to change the name of the Flyin' Saucers to his newly trademarked Frisbee in 1959.

So you tell me where Frisbees really came from.