Toadstool's ??? is an item that appears in Super Mario RPG: Legend of the Seven Stars.
Toadstool's ??? is a rather unique item that can only be encountered once in the game. Toadstool's ??? can be found in Princess Toadstool's room between a blue chair and a fireplace. If Mario manages to find it early in the game, Grandma suddenly rushes over to Mario offering him a Mushroom to forget what he has seen. Later, when Toadstool joins the party, if Mario finds Toadstool's ???, Toadstool will scold at Mario for going through her things. Toadstool's ??? is never seen as Mario keeps it hidden from the player.
Oh, enough speculation, it's a vibrator.