Hi, what is Stupidity?

Hi, my name is Guru Raja Dumfuc Ashol, and lately, I've been hearing things like my cure-all 'Snake Skin and Tibetan White Man-Bear fur oil' is 'quack' and 'fake', and I am 'taking advantage' of people's 'stupidity' by selling self-help books and magical potions that are recommended in my self help books. So, what is stupidity?