後ろ

Etymology
From. Appears in the  of 712. Also cited to a portion of the  dated to 781, with the phonetic spelling. In turn, from. Cognate with 🇨🇬.

Ultimate derivation. Might be from stem us- ("lost; gone, out of sight", as in, older verb ) + locative suffix -ro2 indicating an interior (as in , , , , possibly ). The earliest sense of "somewhere out of sight" also suggests a connection with stem us-.

Noun

 * 1) the back, the behind, the rear
 * , text available at ONCOJ here
 * 1) one's backside
 * 1) one's backside