Warm-heartedness is the quality of being warm-hearted — the kindness itself, treated as a thing you can talk about. It is an uncountable noun, so it takes no 'a' and no plural: 'his warm-heartedness', 'the warm-heartedness of the villagers', 'a story full of warm-heartedness'. You use it when you want to name the quality on its own, usually to praise someone: at a farewell speech, in a book review, in a piece of writing about a place or a person. Be honest with yourself about how rare it is. English speakers much more often say 'she is warm-hearted' or simply 'her warmth' — 'warmth' does almost the same job in one short word and sounds far more natural in speech. Warm-heartedness is a bookish word; it fits a written tribute better than a conversation. Knowing it is useful mainly so you can read it, and so you can see how the -ness machine works.

熱心腸、善良的心意（指「那份好心」這個特質本身）；日常說話台灣人比較會直接說「他人很好、很熱心」，這個字偏書面