What shape would the ethnicon Jewish or Jew have if, just imagine, it were borrowed right from the Ancient Hebrew to the Proto-Slavic? So, we've *jəhūdī: -h- goes to grass, jə- would have faded to j-, thus resulting in *jūd- in Proto-Slavic. As the *-ū- phoneme would've melted under *j- to /i/, it should finally mould in *jīd-. In classical Cyrillics of the Chyov Rousse era this would be written "ид". In the West-Rousse dialects of the modern days Ucrayine it would be additionally plugged with a regular prosthetic j-: /jɪd-/, in Ucrayinian Cyrillics ïд-. Analogously to the most of the ethnicons in the Old (West) Rousse the whole people would be like «идь» or «ïдь» for West Rousse (cf. «русь», «весь», «чудь», «жьмудь» etc), plural in a sence of a social that would be «идовє» («ïдовє» in WR), and an individual would be «идин» with «ïдин» for WR. Also, an adjective would be «идьскъи», WR – «ïдьскъи».
In the herein adopted orthography of Roussish language these forms are written as follows: yidy /jɪdʲ/ "the Jewish People, the People of Judah", yidin /jɪ`dɪn/ "a Jew, a Jewish man", yidofe /jɪ`dɒʋɛ/ "Jewishmen, Jewish assembly or congregation", yidyskuy /jɪdʲ`zsʲkɨɩ/ "jewish or Jewish, pertaining to Jews or Jewish faith and things".
P.S. How would be the Jewish woman or girl called? :??? («идыни», «ïдыни», really? :-\)
їдиня, їдинка, їдка.