expression, godan verb (-su)
rinse someone's back
Used when washing someone's back, typically in a bath setting. Often implies helping someone wash their back, or asking someone to do so.
お風呂で父に背中を流してもらった。
I had my father rinse my back in the bath.
背中を流しましょうか？
Shall I rinse your back?
Compound of 背中 (back) + を (object marker) + 流す (to rinse, to pour water over). Literally 'to rinse the back'.