From the Middle English fon, fonne meaning ‘foolish, simple, silly‘, probably of North Germanic origin; related to Swedish fånig meaning ‘foolish‘, and fåne ‘a fool‘.
An alternative etymology connects the Middle English fonne to Old Frisian fonna, fone, fomne, which are variant forms of Old Frisian fāmne, fēmne meaning ‘young woman, virgin‘; from the Proto-Germanic faimnijǭ meaning ‘maiden‘, from Proto-Indo-European peymen ‘girl‘, and poymen meaning ‘breast milk‘.
If the second etymology is correct, it was then combined with the Old English fǣmne ‘maid, virgin, damsel, bride‘, West Frisian famke ‘girl‘, and the Eastern Frisian fone, fon ‘woman, maid, servant,’ ironically, it also means ‘weakling, simpleton‘.
In short, the modern English word fun probably derives from an early medieval word meaning ‘foolish’, possibly combined with various older words meaning ‘virgin maid’.
So, ‘foolish’ and ‘girl’ come together to spell out “fun” – what else’s new?