It's an object used in prostitution bathhouses.
"The client and companion first get naked. The companion then washes client's body, including his genitals. There is a special chair for this purpose, called sukebe isu which means "erotic chair". This chair has a "U"-shaped seat, which causes the genitals to hang loosely, making them easy to wash. For hygienic purposes, the client usually has to brush his teeth before the session."