Origin and history of fun

late 14c., "deranged, insane;" also "foolish, silly, unwise," from fonned, past-participle adjective from obsolete verb fon, fonne (Middle English fonnen) "be foolish, be simple," from Middle English fonne "a fool, stupid person" (early 14c.), which is of uncertain origin but perhaps from Scandinavian. Related: Fonder; fondest.

The meaning evolved via "foolishly tender" to "having strong affections for" (by 1570s; compare doting under dote). Another sense of the verb fon was "to lose savor" (late 14c. in Middle English past participle fonnyd), which may be the original meaning of the word:

Gif þe salt be fonnyd it is not worþi [Wycliffe, Matthew v.13, c. 1380]

"humorous," 1756, from fun (n.) + -y (2). Meaning "strange, odd, causing perplexity" is by 1806, said to be originally U.S. Southern (marked as colloquial in Century Dictionary). The two senses of the word led to the retort question "funny ha-ha or funny peculiar," which is attested by 1916. Related: Funnier; funniest. Funny farm "mental hospital" is slang from 1962. Funny bone "elbow end of the humerus" (where the ulnar nerve passes relatively unprotected) is from 1826, so called for the tingling sensation when struck. Funny-man was originally (1854) a circus or stage clown.