IPA: /ˈmɔːrnfəli/
KK: /ˈmɔrnfəli/
In a way that shows sadness or grief, often because of a loss.
She looked at the old photograph mournfully, remembering the happy times they had together.
Mournfully → It is formed from "mourn" (from Old English *murnan*, meaning to grieve or feel sorrow) and "-fully" (meaning in a manner full of). The word "mournfully" means in a manner full of grief or sorrow.
Think of feeling deep sorrow ('mourn') and doing so 'fully' — that's why mournfully means expressing grief.