In a carefree, lighthearted, or cheerful manner; without apparent worry or concern.
From 'blithe' (Old English 'bliðe') plus the standard adverbial suffix '-ly.' This is the most common adverbial form and has been in use since at least the 15th century in English literature.
Writers love 'blithely' because it perfectly captures when someone does something happy or carefree without realizing the danger—like when a character 'blithely walked into the trap' in a mystery novel!
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