IPA: /ˈhɑːrtˌrɛndɪŋli/
KK: /ˈhɑrtˌrɛndɪŋli/
In a way that causes deep sadness or emotional pain.
The movie ended heartrendingly, leaving the audience in tears.
Heartrendingly → It is formed from "heart" (from Old English "heorte", meaning the organ that pumps blood and also symbolizes emotion) and "rending" (from the verb "rend", meaning to tear apart) combined with the adverbial suffix "-ly" (meaning in a manner of). The word "heartrendingly" describes something done in a way that tears at the heart, evoking deep emotional pain or sorrow.
Think of something that 'tears' ('rend') at your 'heart' — that's why heartrendingly means in a way that causes deep emotional pain.