the.com/simpleton

Not stupid exactly — just allergic to the second thought.

means A person who is easily fooled or shows poor judgment because they take things at face value without digging deeper.

from Built in the 1600s from 'simple' (plain, unlearned, from Latin simplex, 'single, uncomplicated') plus the mock-Latin suffix '-ton,' likely patterned on names like 'singleton' or 'skeleton' — a jokey, pseudo-grand ending tacked onto a humble word, as if to dress up plainness as a proper title or even a type of person worth cataloguing.

origin
from simple plus mocking suffix, coined 1600s
distinction
fool jokes on purpose, simpleton means it
classic use
Dickens and Twain loved deploying it as insult
modern cousin
basically what airhead means without the glamour
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