IPA: /ˌɪnəˈtɛntɪvli/
KK: /ɪnəˈtɛntɪvli/
In a way that shows a lack of attention or focus.
She listened inattentively during the lecture and missed important information.
Inattentively → It is formed from "in-" (meaning not), "attentus" (from Latin, meaning to stretch toward or pay attention), and "-ly" (meaning in a manner of). The word describes the manner of not paying attention or being careless.
Think of 'not' ('in-') paying 'attention' ('attentus') — that's why inattentively means doing something without paying attention.