Definition of natural fool in English:
natural fool
noun ˌnatʃ(ə)rəl ˈfuːlˌnatʃ(ə)r(ə)l ˈfuːl
historical, archaic A person who is deficient in intelligence from birth; a person born with a mental handicap.
Origin
Late Middle English; earliest use found in The Alphabet of Tales. From natural + fool.