Fun, n.
Etymology: [Perh. of Celtic origin; cf. Ir. & Gael. fonn pleasure.]
Sport; merriment; frolicsome amusement. "Oddity, frolic, and fun." Goldsmith. To make fan of, to hold up to, or turn into, ridicule.
Shades of meaning
Fernald has no entry of its own for fun; he lists it under entertainment, wit.
Where he writes about it:
Cheer may be very quiet, as the cheer of a bright fire to an aged traveler; merriment is with liveliness and laughter; fun and frolic are apt to be boisterous. — from entertainment
Fun denotes the merry results produced by wit and humor, or by any fortuitous occasion of mirth, and is pronounced and often hilarious. — from wit