IPA: //ˈpeɪn.fəl.i//
KK: /ˈpeɪn.fəl.i/
In a way that causes or involves physical or emotional suffering.
She smiled painfully as she tried to hide her discomfort.
Painfully → It is formed from "pain" (from Latin "poena", meaning punishment or penalty) and "-fully" (meaning in a manner full of). The word "painfully" means in a manner full of pain or discomfort.
Think of experiencing something 'full of pain' — 'pain' comes from the Latin word for punishment, and '-fully' indicates the manner in which it is experienced.