köken: From Early Middle Japanese adjective 疑(うたが)わし (utagawashi), with the 終(しゅう)止(し)形(けい) (shūshikei, “predicative form”) ending in し (shi) first replaced with the 連(れん)体(たい)形(けい) (rentaikei, “attributive form”) ending in しき (shiki), and then with the medial /-k-/ falling out to result in an ending of しい (shii). Ultimately from verb 疑う (utagau, “to doubt; to suspect”).