In a manner that causes deep sadness or emotional anguish; painfully and movingly.
From 'heartrending' plus the adverbial suffix '-ly.' This is a standard adverbial formation that converts the adjective describing emotional pain into an adverb describing how an action is performed.
When we add '-ly' to an adjective like 'heartrending,' we're telling someone not just what something is, but how it affects us—'heartrendingly' means the sadness is being delivered directly to your feelings in real-time.
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