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.
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 [Wyclif, Matthew v.13, c. 1380]