jackleg
A total brainless idiot. Someone who thinks they're smart but is really just clueless. Like a goldfish who forgot where the water is.
My brother is a jackleg. He thinks he's a genius but can't even tie his shoes.
My teacher's pet is a jackleg. He thinks he's the best but failed algebra.
My mom's boss is a jackleg. He thinks he's a rockstar but can't even sing.