Fun

Chamber's Twientieth Century Dictionary · 1908 · p. 42

fun, n. merriment: sport.— Be great fun , to be very amusing; In fun , in joke, not seriously; Like fun ( coll. ), in a rapid manner; Not to see the fun of , not to take as a joke. [Prob. a form of obs. fon , to befool. Skeat refers to Ir. fonn , delight.]