In a cheerful, lively, and courageous manner; with a full and happy heart.
From 'heartsome' + adverbial suffix '-ly', which converts adjectives into adverbs describing how an action is performed. This formation follows standard English morphology patterns dating back centuries.
The '-ly' suffix is one of English's most productive word-building tools—you can attach it to almost any adjective and instantly create an adverb! This shows how English evolved a simple machine for creating new words rather than storing them all separately.
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