fondyoudaoicibaDictYouDict[fond 词源字典]
fond: [14] Fond originally meant ‘foolish’, and the likeliest explanation of its rather problematic origin is that it was a derivative of the Middle English noun fon ‘fool’ (its Middle English spelling fonned suggests that it was formed with the suffix -ed, just as wretched was formed from wretch). However, where fon (probably a relative of modern English fun) comes from is another matter.

Links with Swedish fåne ‘fool’ have been suggested but never established for certain. The adjective’s modern meaning ‘having a great liking’, incidentally, developed in the 16th century via an intermediate ‘foolishly doting’. Fondle [17] is a back-formation from the now obsolete fondling ‘foolish person’, a derivative of fond.
=> fondle, fun[fond etymology, fond origin, 英语词源]
fond (adj.)youdaoicibaDictYouDict
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). 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, Matt. v:13, c. 1380]