In a physically attractive way, or generously and liberally.
From 'handsome,' which originally meant 'easy to handle' (hand + -some). Over time it shifted to mean attractive. The '-ly' suffix converts it to an adverb meaning 'in a handsome manner.'
The word 'handsome' originally had nothing to do with looks—it meant something was convenient or easy to use! It evolved to mean attractive partly because attractive things feel good to look at, similar to how we call attractive people 'beautiful' (from an old word for 'good').
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