fun

Concise Oxford English Dictionary © 2008 Oxford University Press:
fun/fʌn/

▶noun light-hearted pleasure or amusement.

■ a source of fun: people-watching is great fun.

■ playfulness or good humour: she's full of fun.

▶adjective informal entertaining or enjoyable. ▶verb (funs, funning, funned) N. Amer. informal joke or tease.

– phrases
in fun not intended seriously.
make fun of tease or laugh at in a mocking or unkind way.

– origin C17 (denoting a trick or hoax): from obs. fun ‘to cheat or hoax’, dial. var. of ME fon ‘make a fool of, be a fool’, rel. to fon ‘a fool’, of unknown origin; cf. fond.