The full etymology is unknown . The oldest known usage of the word is in the 794 text Shin 'yaku Kegonkyō Ongi Shiki . Other old sources include Nihon Ryōiki ( 810 – 824 ) and Wamyō Ruijushō ( c . 934 ) . These oldest sources are written in Man 'yōgana which clearly identify the historical spelling as ki1tune . Following several diachronic phonological changes , this become kitsune .