natural fool
/ˌnatʃ(ə)rəl ˈfuːl/ /ˌnatʃ(ə)r(ə)l ˈfuːl/Now archaic or historical
noun
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.