Ash
You do not recall correctly DY is right. The spelling in yiddish for the word fool is ???.
The word ??? in Yiddish is pronounced Ner since as you correctly point out ayin makes a short e sound.
Words Like ??? and ?? (candle) are usualy spelled the same way since they dont have vowels (in yiddish as you know the letters Aleph, Ayin, Vav … are the vowels, there are no nekudos) so there is no confusion as to how ??? is pronounced you just memorize the vowels. In yiddish the word ??? is pronounced Ner it has a vowel. If you want to write Fool ie Nar the correct spelling in Yiddish is ??? it is ALSO the correct spelling for the word Nur as in only.
Of course if you incorporate a Hebrw/Aramic word for fool ??? it isnt wrong, it just isnt yiddish (Then again the word ?? isnt Yiddish either a candle is ????)