There are several theories: the word may have originated from '鶏居' (bird perch), from "toori-iru (pass through and enter)" or from the way it had been pronounced ("Torana"), to which Chinese characters were applied.
鶏の止まり木を意味する「鶏居」を語源とする説、「とおりいる（通り入る）」が転じたとする説、トラナを漢字から借音し表記したとする説などがある。