na-adjective, noun
stark naked; completely nude
Colloquial and somewhat humorous or childish expression for being completely naked. Often used in lighthearted or teasing contexts.
温泉から上がって、すっぽんぽんで走り回る子供たち。
The kids ran around stark naked after getting out of the hot spring.
彼はすっぽんぽんのまま玄関に出てしまった。
He accidentally went out to the front door completely naked.
The origin is uncertain. It is likely a sound-symbolic (mimetic) word, with the reduplicated 'ぽん' suggesting a light, bouncy, or exposed feeling.