IPA: /ˈsɪmplˈmaɪndɪdli/
KK: /ˈsɪmplˌmaɪndɪdli/
In a way that shows a lack of intelligence or sophistication; naively or foolishly.
He simplemindedly believed everything he was told without questioning it.
Simplemindedly → It is formed from "simple" (from Latin "simplus", meaning single or straightforward), "minded" (from Old English "mynd", meaning mind or thought), and "-ly" (a suffix meaning in a manner). The word "simplemindedly" means in a manner that is straightforward or lacking in complexity of thought.
Think of being 'straightforward' ('simple') in your 'thoughts' ('minded') — that's why simplemindedly means acting in a straightforward or uncomplicated way.
No commonly confused words.