IPA: /ˈrɛstləsli/
KK: /ˈrɛstlɪsli/
In a way that shows a lack of rest or calmness, often due to anxiety or impatience.
She paced back and forth restlessly while waiting for the news.
Restlessly → It is formed from "rest" (from Old French *rest*, meaning to cease work or activity) and "-less" (meaning without) and "-ly" (meaning in a manner). The word "restlessly" means in a manner without rest or ceasing activity.
Think of being 'without rest' ('-less') and acting 'in a manner' ('-ly') that shows you can't settle down.