A person who sells, promotes, or spreads foolish ideas; someone who deals in foolishness or nonsense.
Compound of 'fool' and '-monger' (from Old English and Latin 'mango' meaning dealer or trader), a suffix used for people who trade in or promote something, as in 'fishmonger' or 'warmonger'.
The '-monger' suffix originally meant someone who physically sold goods at markets, but it evolved to describe people who 'sell' ideas—now it's almost always negative, like 'fear-monger' or 'hate-monger,' showing how language captures social criticism.
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