fun Definition

fun
noun

1. Enjoyment; merriment.

Thesaurus: amusement, mirth, pleasure, merriment, merrymaking; play, playfulness, jesting, buffoonery, tomfoolery, high jinks, sport; Antonym: tedium.
2. A source of amusement or entertainment.

adj

1. colloq
Intended for amusement, enjoyment, etc.
Thesaurus: enjoyable, pleasant, lively, happy, merry.

Idiom: for fun

As a joke; for amusement.
Thesaurus: for no reason, for kicks, for laughs, for thrills.

Idiom: in fun

As a joke; not seriously.
Thesaurus: for amusement, not seriously, playfully.

Idiom: make fun of someone or something (poke fun at someone or something)

To laugh at them or it, especially unkindly; to tease or ridicule them or it.
Thesaurus: tease, deride, ridicule, taunt, poke fun at, rag (slang), mock, parody, lampoon, satirize.

Etymology: 17c, from earlier fon to make a fool of.