ブラウザの設定でJava Scriptの使用を有効にしてご利用ください。
出典:Wiktionary
"Why d'you" /ˈwaɪdju/ becomes /ˈwaɪdʒə/ ("whyja") by yod-coalescence (whereby /j/ assimilates to /ʒ/) and reduction of the unstressed /u/ (which becomes /ə/).
whyja