IPA: /ˈhɑrtləsli/
KK: /hɑrtlɛsli/
In a way that shows no kindness or compassion.
She spoke heartlessly to her friend after the argument.
Heartlessly → It is formed from "heart" (from Old English "heorte", meaning the organ that pumps blood and is often associated with emotions) and "-less" (meaning without). The word "heartlessly" means without heart or compassion, indicating a lack of empathy or kindness.
Think of someone who is 'without' ('-less') 'heart' — that’s why heartlessly means lacking compassion.