Hadaka no tsukai is an idea in Japanese culture. It is the idea of spending time together naked. Hadaka no tsukai friendships are not sexual. They are platonic.
A family, a group of housewives from the same neighborhood, a group of businessmen, or a group of classmates might spend time together naked at a sentō bathhouse, at an onsen hot spring, or at a health club. This makes them feel like a group.[1][2]