Na-adjective (keiyodoshi), Noun
utterly stark naked; wearing only one's birthday suit
でも、ジェンの家のベランダにいたヴィンスは上半身裸。いやー、もしかしたらすっぽんぽんだったかもッ！However, Vince, on Jen's house's veranda, was naked from the waist up. He might even have been starkers!
Also written スッポンポン