In fact, it was written as "瞞頭" at first which means to deceive deities to make them believe that is a real human head, pronouncing "mantou" as well.
ちなみに「神を欺き、本物の頭だと信じ込ませる」ことから「瞞頭」と最初で呼ばれ、発音は同じ"マントウ"である。