Etimoloji
A regular shift from Classical Japanese 誣(し)ふ (shifu). Related to 強(し)いる (shiiru, “to force; to compel”). Development Natural progression from Old and Classical Japanese 上(かみ)二(に)段(だん)活(かつ)用(よう) (kami nidan katsuyō, “upper bigrade conjugation”) verbs ending in -u to modern Japanese 上(かみ)一(いち)段(だん)活(かつ)用(よう) (kami ichidan katsuyō, “upper monograde conjugation”) verbs ending in -iru.