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mid-14c., originally "foolish, silly," from past tense of fonnen "to fool, be foolish," perhaps from Middle English fonne "fool" (early 14c.), of uncertain origin; or possibly related to fun.
Meaning evolved by 1590 via "foolishly tender" to "having strong affections for." Another sense of fonne was "to lose savor," which may be the original meaning of the word (e.g. Wyclif: "Gif þe salt be fonnyd it is not worþi," c.1380). Related: Fonder; fondest.