It's rather foolish that Duolingo doesn't accept "foolish". Surely the words have a common origin. It accepted "fooling around" in another sentence.
According to Den Danske Ordbog, it actually comes from French fol !
"fjollet" comes from the verb "at fjolle" which means to play silly. This verb in turn comes from the noun "et fjols" meaning a fool.
OPRINDELSE via oldfransk fol fra middelalderlatin follis 'nar, opblæst person', det indskudte j i fjols har nedsættende betydning ligesom fx i fjog
Translation: "ORIGIN via Old French fol from Medieval Latin follis 'fool/jerk, self-important person', the inserted j in fjols has a derogatory meaning as in for example fjog.
Just wanted to point out it used to be "fol", but it's undergone some phonetic changes and is now spelt and pronounced fou, except before words starting with a vowel.