In a manner showing wholehearted commitment or genuine feeling; with full emotional involvement.
From '-hearted' (a suffix forming adjectives, as in 'kindhearted') + '-ly' (adverbial suffix). The adverbial form emerged to describe actions performed with the emotional quality implied by '-hearted' compounds.
The '-hearted' construction is unique to English—you can't say this in most languages, which means English speakers have a special grammatical way to talk about the emotional quality of actions.
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