IPA: /ˈkwɛs.tʃən.ɪŋ.li/
KK: /ˈkwɛsʧənɪŋli/
In a way that shows doubt or seeks information.
She looked at him questioningly, unsure of what he meant.
Questioningly → It is formed from "question" (from Latin "quaestio", meaning inquiry or a seeking) and "-ly" (meaning in a manner of). The word "questioningly" means in a manner that expresses inquiry or doubt.
Think of someone expressing an 'inquiry' ('question') in a certain way ('-ly') — that's why questioningly means to do so in a questioning manner.