IPA: /ˈmɜː.sɪ.ləs.li/
KK: /ˈmɜr.sɪ.ləs.li/
In a way that shows no mercy or compassion; harshly or ruthlessly.
The storm raged mercilessly, causing destruction everywhere.
Mercilessly → It is formed from "mercy" (from Old French *merci*, meaning compassion or forgiveness) and the suffix "-less" (meaning without) combined with the adverbial suffix "-ly" (meaning in a manner). The word "mercilessly" means in a manner that is without mercy or compassion.
Think of 'mercy' as compassion and remember that 'mercilessly' means acting without compassion.