IPA: /ʌnˈmɜːrsɪfəli/
KK: /ʌnˈmɜrsɪfəli/
In a way that shows no mercy or compassion; harshly or cruelly.
The storm raged unmercifully, causing widespread destruction.
Unmercifully → It is formed from "un-" (meaning not) and "merciful" (from Old French *merciful*, meaning full of mercy) and the suffix "-ly" (meaning in a manner of). The word describes an action done in a manner that is not full of mercy or compassion.
Think of 'not' ('un-') being combined with 'full of mercy' ('merciful') to remember that 'unmercifully' means acting without mercy.