IPA: //ˈfæ.tʃu.əs.li//
KK: /ˈfætuəsli/
In a way that shows a lack of intelligence or thought; foolishly.
He fatuously believed he could win the game without practicing.
Fatuously → It is formed from "fatuus" (meaning foolish or silly) and the suffix "-ly" (meaning in a manner of). The word "fatuously" means in a foolish or silly manner.
Think of someone acting 'foolishly' ('fatuus') — the '-ly' just means in that way. This helps you remember that 'fatuously' describes actions done in a foolish manner.