In a manner full of joy, happiness, or contentment; in a state of perfect bliss.
From 'blissful' (combining 'bliss' from Old English 'blīðe' meaning happy, plus '-ful') with the adverbial suffix '-ly.' The sense evolved from describing fortunate or happy states.
Medieval people used 'bliss' specifically for the happiness of heaven, which is why the word carries such an absolute quality—it's not just happiness, it's perfect happiness. Saying someone is 'blissfully unaware' is almost poetic irony.
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