Heartfelt means deeply and sincerely meant. Heartfelt thanks, a heartfelt apology, heartfelt sympathy.
It attaches to a small set of nouns — thanks, apology, sympathy, plea — and almost nothing else, which makes it a fixed pairing rather than a free adjective. It is written as one word and always goes before the noun.
1adjectiveheartfeltDeeply and sincerely meant.
“He wrote a heartfelt letter of apology to his sister.”
heartfelt is a quality.
Heart plus felt, from the seventeenth century.
From heart + felt.
Etymology from Wiktionary, CC BY-SA